KMS, host keys, client keys, etc.

1. One KMS can host multiple host keys - for example, it can host both Windows 2012 & Office host keys at the same time
2. Host keys are confidential info for companies who bought the license; while client keys are publicly available from Microsoft's website. Client key for a product is same for all companies who chose to use KMS activation.
3. Higher host keys is inclusive in that it covers older, lower products in the same product family. For example, once you install Windows Server 2012 Enterprise Edition, you won't need separate keys to cover standard edition or Windows 2008 servers. The same key covers all.
4. Procedure that can solve vast majority of activation issues in KMS environment
1. make sure DNS is working (can resolve KMS host name correctly), or just use IP address in below commands
2. check if client is using KMS activation
1. slmgr -dlv
2. it should show in output that this is a KMS client. If it's not a KMS type client: 
1. slmgr -upk
2. slmgr -ipk "product key of the OS version/edition". 
   You can google and find the product key
3. check if your client can resolve KMS SRV record
   ping _vlmcs._TCP.yourDomain.name
   if not resolving, you can manually add this record in DNS
   if resolving, your activation should work. Go to the verification step
4. If you don't want to use SRV record, you can also manually tell OS where to find KMS host
   slmgr -skms "A record of KMS host" or
   slmgr -skms "IP of KMS host"
5. verify and active 
   slmgr -ato