I don't quite know what you mean about 'non-sprite object's'. Spritefont is a separate plugin from a Sprite, and in most respects it functions similarly to the text object.
As for 'self', yes, it is a reference to the immediate object. Specifically, a reference to the object said action originated from. If you search for "self" in the page below, there is a little more about it.
https://www.scirra.com/manual/78/expressions