Group Managed Service Account

I found this concept interesting and have even implemented it successfully. Please no links. I have googled this to death.

I have some questions and Windows Server 2012: Group Managed Service Accounts seems to be the likeliest authoritative source.  There are others with different information or more like lack of information.

Question - What is the DNSHostName parameter? Most agree that this is merely the identity plus the rest of the fqdn. Others suggest it should point to the DC. To me the key word is "Host" which suggest it is a computer account and naturally belongs in the DNS but it is not in the forward lookup zone, does not have an A record and I have not gone looking for it.

Question PrincipalsAllowedToRetrieveManagedPassword - some sources suggest that this should be a specific computer account. Can there be more than one defined e.g. SVR1$,SVR2$ If there is only one to a computer account then where is the group part? Can I not create an AD group and set that as the principals allowed? I would like an account for a group of SQL servers, another for web service application pools, one for maybe SCVMM and another for SCCM.

Lastly

Question - What are -ServicePrincipalNames? Something to do with delegation? Of what to what? How would I use this? I think this is a bigger topic so maybe an authoritative link/tutorial would be best.

Oh one more thing. Does the application have to support it? Why would this matter? The service should not care about where the service account is or named only its privileges right? If this is not true is SQL Server 2008 R2 supported?

Thank you for any and all assistance.