CMP is a very feature-rich and flexible protocol, supporting many types of cryptography.
CMP messages are self-contained, which, as opposed to EST, makes the protocol independent of the transport mechanism and provides end-to-end security.
The only other protocol so far using CRMF is Certificate Management over CMS (CMC), described in RFC 5273.
None or any number of registration authorities (RA), can be used to mediate between the EEs and CAs, having both a downstream CMP server interface and an upstream CMP client interface.
CMP messages are usually transferred using HTTP, but any reliable means of transportation can be used.