The "DR" is used by Samsung to indicate that dual-die packaged chips are used.
Conventionally, you can tell the number of ranks in a DIMM by counting the number of chips and their width. So, if you have a module with 36 chips with an x4 (4 bits per chip) designation, then you would expect this to be 144 bits, or 2 ranks.
Because samsung package some RAM chips as dual-die, then it is possible to have a 4 rank DIMM with 36 x4 chips.
For samsung's new stacked die packaging, then they tend to use the terms 2S and 4S, to indicate the number of dies per chip package.