MvdV>> placing a BOM in the middle of the data stream is not forbidden.
MK> I see no reference to it in fts-5003.
You don't see it because it is not there. You also don't see a reference to the soft hyphen, the no-brake space and the delete character. That is because FTS-5003 is not meant to be a documentation for the Universal character set. Same for other character sets like CP437, Latin 1 or ASCII.
MvdV>> We can argue if the presence of a BOM is desirable in the MvdV>> Fidonet environment.
We can, but we won't do it here, it is not a matter for the FTSC. I suggest we move to the UTF-8 echo.