An escalating assertion is a way of expressing yourself in an increasingly firm statement without being aggressive or rude. Example sentence: "I know you want to speak and express your feelings immediately, but I really want to finish my side first before you start speaking again.
"Assertive" is a behavior or description of behavior where a person is able to stand up for their own in a calm, positive manner without being aggressive or forceful.
When being interrupted, one can say "Excuse me, but I have to finish my statement before you can have yours." This is an example of an assertion.