Only what’s documented at the Filtered Transclusion section at Transclusion in WikiText is available for users.
The documentation in the source file is internal only what may be possible with the current regexp.
There is no code for those “may be” possibilities.
IMO low level source code comments should be discussed at GitHub and not here. It’s confusing for users.
As Saq pointed out there is a pending PR at Github, which contains a fix for a rare edge-case. The .classes parameter at the end may be evil.
The PR also contains a proposal to use more parameters for filtered transclusions, which is not possible atm. But there is no consensus yet, what should be implemented and it is low priority too.
This was only an observation and is posted in the Developers Topic. the notes are in the free form text at the top of the code, of our open source project. Presumably it is there as self documentation, and I knew it did not match to the documentation, but did not know anything behind it. Thus commented here.
@saqimtiaz’s answer was a good answer, he directed me to the state of play, your comments @pmario feels a little like censorship.
Curiosity is a healthy attribute. My own curiosity is driven by my knowledge and circumstances which is different from everyone else. I have spent more than 50hours a week focused on TiddlyWiki for a few weeks just now and I have succeeded in finding solutions to long standing issues, some of which many think are impossible or too difficult.
Which I make public or into solutions for the community remain to be determined.
This topic occurred for me because I have a number of solutions I use, that I think the greater community would benefit from and they rely on “filtered transclusions”, so the state of them is important to me.