Condor_submit_dag creates a normal HTCondor submit file, and then (unless the -no_submit flag is given) calls condor_submit on that file. If the DAG file is foo.dag, the submit file will be foo.dag.condor.sub.
Note that condor_submit_dag itself no longer does any parsing at all of the DAG file (a long time ago I think it did a little bit of parsing to check the log file specifications).
nested dags -- eager submit option
deep/shallow options
startup of scheduler universe/daemoncore