What is the difference between a core employee and a contingent employee?

I was just told by the new boss that I'm not a core employee but I've only been out of work for one month. My regular default schedule was supposed to role back in this month but the boss messed things up to where you have to add your availability even though its stated in the default schedule. I really have to start back working but my new boss is making it really difficult. Ive already had a disagreement with my schedule and I don't want to speak with her until I figure things out. Shes also blocking me from transferring.