Even reading the nicely worded explanation from the What are Assignments for instructors? guide - "Assignments include Quizzes, graded Discussions, and online submissions (i.e. files, images, text, URLs, etc.)" - notice the use of the term "online submissions" instead of "Assignment" in this first sentence in order to avoid confusion?
Yet, in the What assignment types are available in Canvas courses? guide it states - "Canvas supports five assignment types: Assignments, Discussions, Quizzes, External Tools, and Not Graded."
All of this gets very confusing and could be fixed by calling Assignments (as in the page) "Assignments" and the other something else. Online submissions sounds good, BUT, since this is also the "assignment" type for assignments that don't get uploaded into Canvas, I don't think it would work.