What screenshots are actually required for submission?

I am confused. The apple guidelines say things like:

If your app indicates that it is optimized for the iPhone 6, at least one 4.7-inch screenshot is required. Up to four additional screenshots can be uploaded. You can rearrange the screenshots when you create or edit the iTunes Connect record.

And

If your app indicates that it is optimized for the iPhone 6 Plus, at least one 5.5-inch screenshot is required. Up to four additional screenshots can be uploaded. You can rearrange the screenshots when you create or edit the iTunes Connect record.

I am about to submit an iPhone game, in my .xcodeproj file I have Devices set to iPhone, so it is supposed to run on iPhone, and iPad in compatibility mode.

What does "optimized for" mean? Where do I set what my game is optimized for? Do I need to provide iPhone 6 and 6 Plus screenshots?

What about iPad screenshots? The compatibility mode screenshots don't look great.