The full documentation of the DBUG library is in files dbug/user.* in the MySQL source tree. Here are some of the DBUG tags we now use:

enter

Arguments to the function.
exit

Results from the function.
info

Something that may be interesting.
warning

When something does not go the usual route or may be wrong.
error

When something went wrong.
loop

Write in a loop, that is probably only useful when debugging the loop. These should normally be deleted when you are satisfied with the code and it has been in real use for a while.

Some tags specific to mysqld, because we want to watch these carefully: