It should be documented. Trial and error shows that @TRUENAME handles directory aliases, quoting/unquoting as necessary. This is useful if you want to send a filename using a directory alias to an external program.
I have been using %@full for that ever since directory aliases were added as a feature. I have batch file substitutes for most of the programs I use that take file names as arguments that, in essence, replace file arguments with %@full[arg] so that directory aliases will be expanded. However, it seems to me that one has to use %@quote to handle cases where there are spaces in the result. For me, in version 27, %@truename does not quote/unquote as necessary.