I'd propose the litmus test for this is "can it initiate action on its own, or does it require your code to do so." In this vein, SQLite is not actually a DBMS

I'd propose the litmus test for this is "can it initiate action on its own, or does it require your code to do so." In this vein, SQLite is not actually a DBMS while Postgres is. The actual Management System parts are things that handle backup, replication, auth, logging, etc. The database parts are dealing with areas that must have some triggering action (calling a stored procedure, running a trigger, selecting a resultset, that kind of thing) for anything to happen. The lines are blurring, yes. What's interesting is the 4GL systems which do the reverse (colocate your apps with your data for speed).