I've been looking at this problem and have been working on an old bundled exportlayers-dwg-BS1192.txt (I am UK-based).
It's mostly working but for one area. Under the Walls category, there are 4 headings that sort/split the primary Walls category by Function. They're some of the few category headings formatted with a : (as in '# Do not remove the colon (:) after certain category names.' from the commented preface section to the file). Probably because the transfer text files were in use some years ago, there are no such headings for 'Soffit' or 'Core-shaft'. I've experimented with adding :5 or :6 (and some more) without success.
Is it possible to add Categories for these functions? I think I prefer to do it with this, as opposed to the Function Modifier for the name - not least because the standard .txt format file doesn't seem to allow modifiers to be added.
I understand that the intentionis that this data be shared only by means of Transfer Project Standards but that doesn't really facilitate making separate export settings from scratch, sometimes based on a non-industry-standard office convention.
Any input gratefully received ...
