These are new abilities addd by Sigil to BS4 to allow it to work better with Sigil. They are not a standard part of bs4.
Here is the only real doc I have on it:
https://www.mobileread.com/forums/sho...d.php?t=265170
There is a simple example near the end illustrating their use. I will try and update the Sigil Plugin Framework docs to include more on them.
KevinH