Hey Bulldog, congrats on the job!
I'm not familiar how per diems work in a clinic, only in a hospital setting, but it might be the same? I would certainly ask!
In a hospital per diems are used as fill in nurses. If they don't need you, they call you off. Per diems are usually the first to go when it comes to being called off.
The pay is usually a bit more because you don't get sick,vacation or insurance.
I worked per diem for years. I got better pay and didn't need the benefits.
Yes, you fill out a W-2. No difference there.