Create bearer request message is sent by PGW to SGW and SGW to MME as part of either UE requested bearer resource allocation procedure or UE requested bearer resource modification procedure. I am covering here only LTE nodes. This message can also sent by PGW to SGW as part of network initiated dedicated bearer establishment procedure. A dedicated bearer might be GBR or non-GBR based on QoS requirement. One more thing, every dedicated bearer is linked to a default bearer.