MGRL Selection Process
Cisco CallManager uses the MRGL concept in order to select resources. The selection depends on the geographical assignment of the resources
MRGs are logical groupings of media resources. A single MRG can contain hardware conference resources, software conference resources, transcoder resources, MOH servers, and software Media Termination Points. An MRG has no user-defined order. All resources in an MRG are considered equal. Therefore, Cisco CallManager loads share between resources of each type in one MRG.
When transcoding is used with a conference, the transcoder is selected based on the MRGL of the Conference Bridge.
Note: You cannot explicitly configure an MRGL for a Conference Bridge. Therefore, the MRGL is taken first from the Device Pool, and then from the MRG default pool.
When a phone is put on hold, the MRGL of the device that it put on hold (could be a gateway for offnet calls) determines which MOH server is used to play music to the held device.
Conference Bridges are chosen based on the MRGL of the conference controller (the party that initiates the conference).
If a call goes out through a gateway, and Media Termination Point (MTP) is required. The MRGL of the gateway is then used to select the MTP.
MRGLs are an ordered list of MRGs. All resources in one MRG must be exhausted before Cisco CallManager attempts to use a media resource from another MRG in the same MRGL.
MRGLs can be associated on a per-device basis, which means that you can give specific devices access to media resources on an individual basis. A second MRGL can also be configured at the device pool level.
If a device has an MRGL configured at the device pool level as well as on the device itself, the MRGL configured at the device level is searched first, followed by the MRGL on the device pool.
The last MRGL is the default MRGL. A media resource that is not assigned to an MRG is automatically assigned to the default MRGL. The default MRGL is always searched and it is the last resort if no resources are available in the device-based MRGL and the device pool MRGL or if no MRGLs are configured at any level.
https://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/213261-understanding-media-resource-groups-and.html