Because of legalities AND compatibility/contamination issues, you'll find that any real AC tech has to have a separate set of gauges and hoses, and a refrigerant recovery tank, for EACH gas that they use. So if they use R12, R22, R134a, R410...that's four whole sets of equipment
plus four recovery tanks
that they have to carry around.
Which is another reason that many techs just stop servicing R12 and R22, too much stuff to haul around for too few jobs.
And then of course there are the real commercial
techs, who have to work
on ammonia based systems like your supermarket uses. That stuff will bite, big time.