I mostly use A/B for parking during a trip, but have also used E/F/G. I have never tried to use M. You do not need to pre-book A-G, but the available parking spaces are limited. Last year I had two departures where all parking lots (except C) were fully occupied and you could only enter a parking lot, if someone was leaving, or if you had reserved online. I hope that there will by enough capacity when Skypark opens...

The algorithm how and when they sell spaces for pre-booking is also not clear to me. I do not remember that I was offered F/G as an option when checking online, at least in the last couple of years.