What is the difference between on-call, per diem, rotating and registry, in terms of work hours/availability?

Question

What is the difference between on-call, per diem, rotating and registry, in terms of work hours/availability?

Answer 1

On-call: Ya hire that person upon requesting to work for a project & ya only pay him after he had accomplished that certain project.

Per Diem: Is paying his hours of work on a daily basis.

Rotating: Ya pay his work for a period of time with breaks in between those periods without work & pay.

Registry: Ya pay the person only when he registers for work, that is if required, otherwise, no work registry, no pay.