I don’t know that the destination value’s scope has ever been clearly defined. My impression is that some routing engines behave as though a destination edge requires the route to begin or end on that edge, whereas other routing engines consider all the adjacent destination edges together. The latter is definitely true of routers that treat private as a sort of destination, returning a route from your current location even if you’re trapped inside a gated, private neighborhood. This is the only way that we’d be able to model some kinds of restrictions, for example when a town declares all its streets off-limits to through traffic, or an entire network of highways and streets around an airport is for airport traffic only.