The Unit Separator explained
In Videotex the Unit Separator is followed by one or 2 characters. If the first character is in the Range of A..., it's used as a row indicator and the second character is the column indicator, so it's a "Goto XY" command. If the first character is below "A", it indicates what layer you want to talk to.
Defined layers include a colour palette definition, the definition of user defined characters or vector graphics. Standardisation of audio doesn't seem be be complete, but there are fragments in the standards.