Onto is a preposition of movement, where a subject moves relocates to a 'flat' surface, either actually or metaphorically: He jumped onto the train. More features were added onto the policy. On to is an adverb + preposition, where the adverb indicates continuance and the preposition is one of movement in a direction toward something: The meeting moved on to the next item of the agenda.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.