Comment by jka

Sure; let's take the example of a code file, where semantic validity could make the difference between a correct program and one that doesn't compile.

Validity checks could include a syntax parser as a first-pass, followed by unit test as a second pass. If any of the verification steps fail, then the application can display that there is a problem with the file (and hopefully some context).

The authorities in that example are the software that checks the program; it can run locally for each participant in the session, or there could be a central instance that runs them, depending on the preferred architecture for the system.

None of the above necessarily requires discarding edits; but in some cases participants might choose to undo edits that caused problems, in order to get back to a valid document state.