Do i have to buy a license no matter which implementation i use?

[oliverunger]

Related to the wiki stating:
Schematron is part of the ISO 19757 standard "Information technology — Document Schema Definition Languages (DSDL)", Part 3 "Rule-based validation — Schematron". Until version 2016 the standard was free, since the 2020 version the standard needs to be bought.

https://github.com/phax/ph-schematron/wiki

[phax]

You only need to buy the Schematron specification from ISO if you want to read it. For the usage of the software you don't need it