The Seat License is = to the number of users, so for your an Enterprise with 900 users you might order a 1,000 "Seat License". Licenses are generated for requested Cisco CallManager nodes (servers in a Cisco CallManager cluster - with seats) and the phones that are associated with those nodes (DLU's).
Just to add to what Rob has mentioned, typically seat licenses are associated to your UCM cluster and is dependant on the MCS server.
Following are the seat licenses available -
LIC-CM6.1-7816= License Unified CM 6.1 7816 Appliance, 500 seats
LIC-CM6.1-7825= License Unified CM 6.1 7825 Appliance, 1,000 seats
LIC-CM6.1-7835= License Unified CM 6.1 7835 Appliance, 2,500 seats
LIC-CM6.1-7845= License Unified CM 6.1 7845 Appliance, 5,000 seat
LIC-CM6.X-2500= License Unified CM Additional 2500 Users
If you observe 7816 mcs server can support up to 500 phones hence you have the license of 500 seats. Similarly 7845 can support up to 7500 users. You have two licenses for 7845. If you are planning to have not more than 5000 users on a 7845 cluster then you can go for "LIC-CM6.1-7845=" license, however if you plan to have more than 5000 phones then you need to additionally order "LIC-CM6.X-2500=" license. Please note that For all the MCS servers apart from 7845 the number of seat licenses are available for the max phones supported. For exmple for 7835 cluster which can support 2500 phones, you can't order a 1000 seat license even if you plan to have no more than 1000 phones registered to the cluster.
Coming to DLUs, it's the number of licenses required per Phone to register to the UCM.
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