DEBUG - display currently defined variables.

$*$DEBUG

can display information about various items found in an EXEC file.

$*$DEBUG Vars

displays the types and values of all currently defined variables.

$*$DEBUG Args

displays the types and values of all current command arguments.

$*$DEBUG Labels

displays information about all known statement labels.

$*$DEBUG Files

displays information about all known EXEC files.

$*$DEBUG Commands

displays information about all user-defined commands.

$*$DEBUG All

displays information on all of the above items.
If you simply use

$*$DEBUG

without specifying what kind of information you want, EXEC displays information on variables and arguments.
You can specify ",+" on the end of any $*$DEBUG command, as in

$*$DEBUG Commands,+

This tells EXEC to display more than its usual amount of information on the selected items.