Erie doctrine says:

A case may go to federal court on one of two bases.

(1) The first, is if the anchor/main case is based on a federal question. (Pure federal controversy) In this case, the court will apply federal substantive and procedural rules. (Procedural as in the FRCP)

(2) The second is on diversity jx. Diversity means its really a state case but either the plaintiff files in fed first or the defense moves it to fed court. Minimum requirements must be met (amount in controversy exceeds a certain dollar amount etc, parties have minimum contacts with forum state, convenient for witnesses, situs of controversy etc). When a case goes to fed in diversity jx, then the court will apply federal procedural law and state substantive law.

//
//
The big difference between the two is the application of which procedural and substantive law.
//
//