There are two interfaces (Gx and Gxx) interfaces from PCRF, one ends at PCEF/PGW and second one on SGW/BBERF. Here Gxx is represting Gxa and Gxc interfaces. If access network gateway is 3GPP standard node then interface is known as Gxc and if access network node is trusted non-3GPP then interface is known as Gxa. when PMIP protocol is being used between PGW and SGW then PCRF uses Gxc interface with SGW for policy and charging control. SGW has a component BBERF(bearer binding event reporting function) which interact with PCRF over the Gxc interface.
There are common messages exchange occurs over the Gx and Gxc interface. Credit-Control-Request, Credit-Control Answer, Re-authentication Request and Re-authentication Answers.Message names are same but application id is different.
Dedicated bearer creation doesn't differ since message exchange over the Gxc and Gx interface is same CCR and CCA. Please correct me if I am wrong.