They are the orginal receiver that came out many years ago that allowed you to pick up everything that was on the satellites. HBO, Movie channel, etc. Then scrambling came next and they were used on a subscription basis. Still in use today with modification on one sat for pay for TV programs. Many use them today to move their big dish and to make polarization adjustments. That is what I use my 920 for.
The DSR410, DSR905, DSR920 and DSR922 are able to sub to H2H services. Check out the C band area or Google 4DTV for more info! There are also GI/Motorola "DSR" series satellite receivers used in commercial applications. Some of the commercial series models are used by hobbyists like us! I have a DSR-4810 (4:2:2 capable) that I am going to mess around with this winter to see what it will do!
What model(s) are you curious about??
H2H stands for Hits-2-Home, which is Comcast's Headend In the Sky. The channels are uplinked to AMC18 at 105W by Comcast on C-Band. This service is receivable on a 4DTV or DSR series receiver. The programming is meant for small cable systems, however this service is sold to the TVRO market by The Programming Center and Skyvision.