These are for providing media resource functions like transcoding, conferences etc. Normally most of these are required when you go use different codecs (like G729 etc.) and have multiple sites. But you can go through the media resource chapter of the SRND to get a thorough understanding of the function of each type of media resource, here: http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab10/collab10/media.html
-Terry
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