I am trying to understand the operation of the Dual_EC_DRBG. I'm reading the formal specification (SP 800-90) and can't seem to find a definition of the $\phi$ function used throughout the definition that I can grasp. The definition I found is on page 65 of SP 800-90A. Just to be clear, I'm referring to the $\phi$ found in this diagram.
Can anyone explain what this function does exactly? Is it a common function in elliptic curves? Something in number theory that's just over my head? Any help would be much appreciated.