2. "It is the job of this person to make the coffee"
3. "The people whose job it is to catch criminals are the police."
4. "It is the job of the police to catch criminals"
There's a difference in meaning between these two ways of saying something. In sentences 2 and 4, it sppears that that is the person's/police's only job. Sentences 1 and 3, however, merely point out whose job something is.
3 and 4, for example, are the answers to the following two questions, respectively:
5. "Whose job is it to catch criminals?"
6. "What is the job of the police?"
I'm not sure what rules you want to know about.
Student or Learner