Exact Meaning of MySQL's Foreign Key 'on delete restrict' Clause

Question

I have two MySQL tables: collections and privacy_level.
I define them with a foreign key relationship as such:

Answer 1

ON DELETE RESTRICT means you can't delete a given parent row if a child row exists that references the value for that parent row. If the parent row has no referencing child rows, then you can delete that parent row.

ON DELETE RESTRICT is pretty much superfluous syntax, because this is the default behavior for a foreign key anyway.