Some Planes Show as SimObjects and Some Don't

I’m doing a little SDK’ing (just for fun, not going to publish) and I noticed that in the SimObjects, Asobo’s planes can be selected while some of the planes in my Community folder show there and some don’t. What’s the magic setting that determines whether a plane shows as a SimObject? I searched for this but could not find it in the forum, so apologies if it’s already been asked and answered.

Thanks.

Also very curious about this.