Terms of sale and license

For the software Stack by F³ Lab, sold and licensed at stack.f3lab.tools. In force from 5 September 2026.

1. Who sells

Seller: The Zone Modest Reszewicz (sole proprietorship registered in Poland under the name Modest Reszewicz)
Tax ID (NIP): 6932129511 · EU VAT PL6932129511
REGON: 542483912
E‑mail: support@f3lab.tools

These terms govern the use of the site, the registration of the Free edition, the purchase of a Pro license, and the use of the software itself. A customer who buys as a consumer keeps every right the law of their country of residence gives them; nothing here reduces those rights.

2. Words used here

Stack — the desktop application Stack by F³ Lab for Windows 10/11 (64‑bit), for tolerance stack-up analysis.
License file — a signed file (license.lic) issued for one computer, which the application needs to run.
Machine code — a code in the form XXXX‑XXXX‑XXXX‑XXXX that the application derives from the computer it runs on and shows in Help → About → License. It identifies the computer, not the person.
Free edition — the license issued at no charge after registration: every feature, a “not for commercial use” watermark on the calculation sheet and on every report, at most 20 chains per project, no expiry.
Pro license — the purchased license: every feature, no watermark, no chain limit, reports that carry the licensee’s name, valid for one year from issue.
Customer — whoever registers the Free edition or orders a Pro license, whether a consumer or a business.

3. What is sold

Stack is digital content delivered electronically. Nothing is shipped. The installer is downloaded from this site; the license file is issued for the customer’s machine code and sent by e‑mail. The application works offline and needs no internet connection to run or to activate. Requirements: Windows 10 or 11, 64‑bit.

The application is provided as described in the manual and the white paper. Its results depend on the data the user enters; the seller does not warrant that any particular design is correct, only that the application computes what those documents say it computes.

4. The Free edition

Registration on the Free license page requires a name, an e‑mail address and the machine code; a company or school name is optional. The file is issued at once and also sent by e‑mail.
The Free edition is for non‑commercial use: learning, teaching, evaluation, private projects. Using it for work that is paid for or that produces documents handed to a customer or an employer requires a Pro license.
One Free license is issued per computer. Up to three computers may be registered to one e‑mail address; write to support@f3lab.tools for more.
The seller may refuse or revoke a Free license issued on false data or used commercially. A revoked license stops working at the next start of the application.
No contract of sale is concluded for the Free edition; it is a free license granted under section 7.

5. Ordering a Pro license

Prices are shown net, in PLN or EUR depending on the customer’s country, and VAT is added at the rate of the customer’s country before payment. The total shown at checkout is the amount charged.
To order: add Stack Pro to the cart, give your name, address and e‑mail, optionally the machine code (it can be provided later) and, when renewing, the renewal code from the reminder e‑mail, choose a payment method and confirm. The order confirmation e‑mail is the moment the contract is concluded.
Payment is by card, Przelewy24 or BLIK, handled by Stripe. The seller never sees card numbers.
An invoice is issued electronically to the e‑mail address given in the order. A business customer should enter the company name and VAT number in the address fields so that they appear on the invoice.
If the machine code was not given at checkout, the confirmation e‑mail carries a link to a page where it can be entered. The license cannot be issued without it.

6. Delivery of the license

The license file is issued within two working days after both the payment is received and the machine code is known, and is sent by e‑mail as an attachment. It is imported in the application through Help → About → License → Import license file. A renewal license starts on the day the previous license ends, so renewing early costs no days.

7. License terms

The seller grants the customer a non‑exclusive, non‑transferable license to install and use Stack on the one computer identified by the machine code, for the term stated in the license file: one year for Pro, without expiry for Free. The customer may:

use the application for any lawful purpose within the edition’s scope (Free: non‑commercial);
keep backup copies of the installer and of the license file;
use the reports, drawings and files the application produces without restriction; they belong to the customer.

The customer may not:

share the license file, or use one license on more than one computer at a time;
remove or circumvent the license check, the watermark or the chain limit, or modify, decompile or reverse engineer the application beyond what the law expressly allows;
rent, lease, sublicense or resell the application or a license.

A new computer. A license is bound to the machine it was issued for. When a customer replaces a computer or reinstalls Windows during the license term, the seller issues a replacement file for the new machine code on request at support@f3lab.tools, at no charge, within reason; the old file stops being valid.

Updates. Updates published during the license term are included. The application checks for updates only when the user asks it to (Help → About → Check for updates) and downloads nothing on its own.

Intellectual property. Stack, its documentation and this site remain the property of the seller. The license grants use, not ownership.

8. Right of withdrawal (consumers)

A consumer may withdraw from a distance contract within 14 days without giving a reason. For digital content that is not delivered on a tangible medium, the law provides that this right is lost once delivery has begun with the consumer’s express consent and acknowledgement that the right is thereby lost.

For a Pro license this works as follows: the license file is issued only after the customer has provided the machine code. By providing the machine code and requesting the file, the consumer expressly asks for delivery before the end of the 14‑day period and acknowledges losing the right of withdrawal from the moment the file is issued. Until the file is issued, the consumer may withdraw by writing to support@f3lab.tools; the payment is refunded within 14 days by the same method it was made.

The Free edition is granted free of charge and can be given up at any time by deleting the file.

9. Complaints and conformity

The seller is liable for the conformity of the digital content with the contract under the Polish Consumer Rights Act, for two years from delivery for consumers.
If the application does not work as described, write to support@f3lab.tools with the order number, the machine code and a description of the problem; the Help → Log tab of the application produces a diagnostic file that helps.
Complaints are answered within 14 days. Where the content does not conform, the seller first brings it into conformity (a fix, an update, a replacement license); if that is impossible or unreasonably costly, the consumer may demand a price reduction or withdraw from the contract.
For business customers, the seller’s liability is limited to the price paid for the license, and statutory warranty is excluded to the extent the law allows.

10. Out‑of‑court dispute resolution

A consumer in Poland may use the out‑of‑court procedures of the Trade Inspection (Inspekcja Handlowa), the permanent consumer arbitration courts at its voivodeship inspectorates, and the free help of the municipal or district consumer ombudsman. A consumer in another EU country may turn to the alternative dispute resolution body competent in that country. The seller will take part in such a procedure when asked; the first step is always an e‑mail to support@f3lab.tools.

11. Personal data

How the data given at registration and at checkout is used, kept and protected is described in the privacy policy, which forms part of these terms.

12. Final provisions

Polish law applies to the contract; a consumer is also protected by the mandatory provisions of the law of the country of their habitual residence.
The seller may change these terms for licenses issued in the future, for example when the editions, the prices or the law change. A change is announced on this page with its effective date and does not affect licenses already issued.
Where a provision is found invalid, the others remain in force.
These terms are available at all times at stack.f3lab.tools and can be saved or printed.