CUCM treats the media resources you add to the cluster like a bowl full of goldfish: it simply grabs the first one it can from the bowl when they are needed (this is known as the NULL Media Resource Group). In this Nugget, Jeremy walks through the methods of categorizing, grouping, and prioritizing which media resources CUCM should use FIRST when they are requested.
MRG + MRG
MRGL is assigned to a Device or at a Device Pool level.
Media Resource Group – (All available Media Resources are listed)
Media Resource Group List – A list of groups