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What do they not cover, and how do you sign up, I live in Ajijic

They have a very comprehensive booklet you can pick up at the office on Degollado in Chapala that goes into detail about the coverage or non coverage.
Too lengthy to go into it here and it changes frequently and policies are not always uniformly carried out or applied, so you have to keep yourself re-informed on a regular basis.
You sign up at the same location, bringing the usual documents that are needed for any government inscription or license.
There are no fees or charges.

Can I sign up at the Ajijic office at six corners

No. Only at Centro Salud in Chapala or the hospital.