I would say that they are LARGELY interchangeable. Some companies just use them differently. I was accustomed to "staff" meaning the people who reported directly to a person. "Nancy's staff has really grown since her area has taken over responsiblity for regulatory compliance."
However, I've since worked places where "staff" simply means "all the people who work there."
1/What I don't know is when they areinterchangeable, when they can't beinterchangeable, clearly.
2/Besides, in library personnel staff, either personnel or staff is redundant (because both staff and personnel here refer to the same thing)? Is it right? If n
It's not what you are asking, but I suspect that you will find the word 'employees' used in many places where an English learner might try to use 'staff' or 'personnel'.