AXL is the interface through which UCCX syncs/pull configuration from CUCM. When you configure your call control group, jtapi trigger etc, this is the interface through which uccx configures the devices, on cucm. Its as simple as that.
There are two types of users you define in uccx..
1. RMJTAPI user: This user is used to login to cucm and all your call control devices ie cti_route point (jtappi trigger) are associated with this user
2. RMCM user: This user is used to monitor agent states by uccx so as to inform cucm of what to do with this user. All agent phones must be associated with this user.