"Service subject" has no meaning to me in American English. British English perhaps?

I looked up the phrase and found "Teaching mathematics as a service subject" And this makes a lot more sense. So a teacher teaches subjects. This is about teaching the subject of "Mathematics as a service" So, the subject is about how to use Mathematics to do something.

Sounds like jargon to me. My guess is what they mean is that the students' primary focus is a subject other than maths, but they need it for what they are actually studying, for example physics or engineering.

I guess service subject kinda means main subject? Coz the students who are major in math has to treat maths study as their main task

the context could help, it is the following: 'This is true whether the student are specializing in mathematics or are registered in a program for which mathematics is a "service subject" '