Comment 3 for bug 2024343

IIUC https://gitlab.com/fedora/legal/fedora-license-data/-/issues/154 concludes that the most likely interpretation of the (unclear/ambiguous) terms are that only *free of charge distribution* is allowed ("signifies that you can only distribute without charging money for copies").
Not a problem for lxml per se in my humble opinion, but obviously(?) for distributors - IANAL.

Unfortunately the Schematron ISO "open" standards (2006, 2016, 2020) documents aren't available free of charge any more, so no easy way to assess if a properly open source licensed from-scratch implementation of a schema-for-schematron could be feasible. Rick Jelliffe epresses his discontent on this here: https://www.schematron.com/standards/iso_schematron_2020_released.html.

https://www.schematron.com/home/open_licenses.html states that "Schematron and RELAX NG schemas for ISO Schematron provide and open and free license in their headers; the license is the same as the ISO 8879:1986 standard public entity sets for SGML with an additional clause requiring that modified files state that they are modified.". Which seems to be what was originally included in lxml at isoschematron inception time (https://github.com/lxml/lxml/commit/a9144fcdde863301564b2a4b63c47ba2681dc076#diff-7e50a93c3193a8d1a413707d120dc08b4bf39aaccb5b02fd5394bf2e3607d385) - albeit arguably even more unclear/ambiguous.