Writing for iMore, Serenity Caldwell has a great overview of On-Demand Resources and how they’ll work on tvOS:

Instead of making the user download 4GB off the bat, you slice up your app into a bunch of sections, called tags. You include the essential parts of the app—loading and launch screen, scores, settings, and the first five levels—in that 200MB bundle.

Other levels and assets are split into multiple tags that range in size from 64MB to 512MB. If you sliced up tags that all sized out to 100MB for your game, for instance, you’d have 38 additional items for download once a user installs the game. Those don’t come all at once, however: They’re called on-demand, when a user needs them.

(Extremely geeky thought: I wonder how this could affect the speedrunning community and level-skipping glitches if similar technologies are adopted by more platforms.)