Hi Regina Henschel,
The "character unit" used here comes from RTF and binary MS-DOC and refers to twips. I had this question back in 2011 about RTF/MS-DOC but didn't realize its in ISO 29500 as well. I have to confirm first even thought I believe this is the case and then I'll see if the ISO Standard committee would agree to add a note.
Thanks for bringing this to our attention.
Best regards,
Tom Jebo
Microsoft Open Specifications Support