Before posting any operations to a particular interface, the caller must first open a context for that interface. This is a mechanism by which an interface can differentiate between two different callers (ie, if operations are being posted by more than one thread or more than one higher level component). This context is then used as an input argument to every subsequent post and test call. In particular, it is very useful for the testcontext() functions, to insure that it does not return information about operations that were posted by a different caller.