A separator is not really needed. Anything between "REM" and traling "START LBA NUMBER" is to be treated as a plain comment or a COMMAND, which will be recognized if its called "SESSION NUMBER" or "LEAD-IN" or "LEAD-OUT" or whatever commands you want to have there.
I think it should start with a real command and require input of a number, for uniformity and to allow other uses besides dreamcast.
REM <COMMAND> <OPTIONAL DESCRIPTION> <START LBA NUMBER>
REM AREA "SINGLE-DENSITY" 45150
REM START "SINGLE-DENSITY AREA" 45150
Where the " " are optional and the last number is assumed to be the start address?