TM30. You have to do it if you will need any services from Thai Immigration or you are entering a renting agreement, then the landlord have to file TM30 for you, you do nothing.
Say, your cousin or friend come to visit you in Thailand, in a condo/house you already rent or own, no action, no TM30 for them.
I was just told by a long term Thai resident (an American), after an uporoar from the tourism industry, Thais scrapped the hotel obligations to fill in your TM30, 2 years ago.
Indeed, 40 million tourists a year, many stay in hotels for a night or two, change and travel further, what Thai workforce is needed to punch in all those TM30s.