I cannot find any mention of this in the documentation
But while it may not be documented putting documentation in there for a property is retained as a note as part of the property
there are other items where this could be used but isnt
simply put notes in one and save as text and you will see it like
#tag Property, Flags = &h0
#tag Note
foobar
#tag EndNote
Untitled As Integer
#tag EndProperty
the editor itself could be set up to not try and syntax color the notes you put in there (which is why you see keywords get upppercased like they are
constants dont support anything like this