So is a polite legal threat assertive or aggressive?
edit: I'm saying - where you draw the line is about things like whether it's competent, and whether it's "OK" within the contex. A polite "excuse me" is both competent and almost always "OK". A polite legal threat might not be "OK" (and it might not be competent either) so whether you judge it to be aggressive will depend on your point of view.
It's not a line. Something can also be both aggressive and assertive. That they are different does not make them mutually exclusive.
One can also be awkward but not polite, polite but not awkward, and politely awkward. They're just different characteristics.
One can be assertive without being aggressive and aggressive without being assertive. They are different characteristics.
Assertive and not aggressive - deciding to speak up instead of being silent. "Excuse me, can you explain this decision to me?"
Aggressive and not assertive - being passive-aggressive, giving people the silent treatment / cold shoulder / "cancel culture", and similar behaviors.