I thought I am clear on this but not sure after worked on a customer CUCM server...I will try my best to explain. Lets only talk software based media resources (Annunciator, MOH, Conference and MTP).
The simple version of my confusion: When configuring any of the media resources, e.g. Annunciator, there is an option to select which device pool to put the media resource in. But what does that do really?
Here are my understanding:
Media Resources belong to None media resource group by default and will need to be put into Media Resource Group, which will be then put into Media Resource Group List in order to control which devices can use them. The phones will grab the Media Resource Group List configure from the device pool it belongs to if not specifically configured under its own configuration page. Each device pool will have specific Media Resource Group List configured, by default it is using None.
So lets say there are 2 Device Pools on a centralized CUCM: Pool1 (East) and Pool2(West) with MRGL1 and MRGL2 respectively. Annunciator is configured to use Pool1 instead of default Device Pool and add Annunciator into media resource group lists. What does that Pool1 device pool configuration do to the Annunciator really? Not for access control, I guess?
What if I just create a device pool, MediaPool, and assign all the media resources into that pool?