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.
TLDR : Answer Summary
The TM30 is a notification requirement under Thai immigration law that mandates the owner or host of a property to report the presence of any foreigners staying at their address within 24 hours. This rule is essential for anyone needing services from Thai immigration or for those entering rental agreements. While hotels are still required to submit TM30s digitally for their guests, private hosts must report their foreign guests manually. The enforcement of TM30, however, varies locally, with some foreign nationals experiencing strict enforcement, while others may not be regularly impacted.