My understanding now is 2 date windows per year. Then you get one transfer for free to play the next year. Then if you don't transfer as a new graduate again then you have to sit a year. If you are a graduate at the latest school then you get another free immediate year to play.
Other side is if player is a freshman but plays more than 4 games he can still transfer once and play. If he transfers again before graduating then he has to sit a year at new location. If he sits there and plays for a year but graduates, then he could go to new school for different degree and play immediately.
Hope that makes sense, but its very lean on rules nowadays.
Here is a fairly concise explanation. Fun stat, 20.9% of FBS starters were transfers this year. Also, tampering is widespread and obviously unenforceable. Back channel communications aren’t really back channel anymore.