Board Thread:General Discussion/@comment-25142096-20141011054905/@comment-1824272-20141011183849

This is my theory:

Four-star aircraft are tournament prizes or random drops.

Three-star aircraft are random drops that are related to another series/collaboration event. (If iDOLM@STER aircraft are released, I would expect them to be three-star.)

Two-star aircraft are the aircraft that I consider "four-star rejects", as in, they could have been four-star aircraft but they were intentionally marked down and made easier to obtain. In other words, two-star aircraft are essentially four-star aircraft that are easier to obtain. This includes the tournaments they're involved in.

One-star aircraft are throwaway aircraft given to everyone, either as a gift or in a very very easy challenge.