I did not "write" any of the regulations - I just do a Copy & Paste of what I find online. I do not "know" what the laws are - only what I read and have heard from others. I have GOOGLED to find the actual law from a Mexican Gov't website but have not found it. Easy to get one online at the website at the bottom of this post.
https://www.bajabound.com/before/permits/visa.php
https://espanol.bajabound.com/before/permits/visa.php
What is the tourist card / tourist permit?
According to the National Immigration Institute, a tourist card (FMM) is an "admission document" issued to vacationers of certain nationalities. US and Canadian citizens who travel to Mexico for vacations can obtain the Tourist Card (FMM) in a few minutes at a Mexican immigration office at the border with a passport or a passport card. The tourist card (FMM) can be issued for up to 180 days and is issued per person including minors.
When do I need a tourist card / tourist permit?
People entering Mexico as tourists need the tourist card (FMM). It is important to note that in Baja, although you can take your vehicle to the end of the peninsula without a vehicle permit (see import regulations for vehicles ), you must have the tourist card (FMM).
Who can get a tourist card / tourist permit?
If you are a US citizen or Canadian citizen traveling to Mexico for tourist reasons, you may only need a tourist permit (FMM). For citizens of other countries it is better to contact the Mexican Consulate for information on documentation requirements. You can see a list of countries that require and do not require an official Visa here
What kind of documentation do I need to obtain a tourist card / tourist permit?
If you are a US citizen or Canadian citizen, you will need to have a valid passport to present to Mexican immigration officials to obtain the tourist card (FMM).
Lots of Govt stuff about FMM at this website.......
https://www.gob.mx/busqueda?utf8=%E2%9C%93&site=inm&q=Forma+Migratoria+M%C3%BAltiple