Applicability is the usefulness of something for a particular task. Hammers have great applicability for driving in nails.
When something is applicable, it is suited to something or useful for a task. The applicability of a thing refers to how useful it is in a given situation. On a resume, talking about your favorite movie probably would have no applicability: it’s pointless. A plunger has great applicability in a bathroom, but no applicability anywhere else. You can think of applicability as usefulness plus timeliness. If something has applicability, it will come in handy at the moment it is needed.
relevance by virtue of being applicable to the matter at hand