DirecTV is getting a little funky because of the way they are rolling out 4K. In our part of the country if you are asking for the latest you will probably get the following:
HR54 as the server
C61 for wired clients
C61W for wireless clients
C61K for 4K
There is still a limitation that you can only have 1 C61K per system. And I believe there is a total max of 4 clients in a system.

We recently had an install with the new genie server that I was told we didn't have in our area yet. That piece is the HS17 which is a true server but not a TV output. Supposedly it can allow 6 C61Ks at a time. The system we saw had C61Ws as the client didn't have any 4K TVs.

Of course depending on the crap-shoot of what the installer has on the truck you may get a 10 year old HR24 as well.