Plumbers used to do sprinkler/ standpipe work, and fitters did gas piping. The unions made a deal, Union Local 638 would do heating and fire suppression, give gas work, as well as plumbing, to the plumbers' unions
Around the 1990s, there was a test with no practical for sprinkler/fire suppression, and plumbers could legally install up to 100 sprinkler heads
I decided to get my master fire suppression piping license so I could legally do both, including heating, boiler installations, and all types of service work.
Crazy how much more fire suppression insurance is as opposed to plumbing/ga