What does DR mean on HDPE pipe?
DR, or dimension ratio, is the pipe's outside diameter divided by its wall thickness. A lower DR is a thicker wall. Because it is a ratio rather than a measurement, every size of pipe at the same DR carries the same pressure rating, which is the whole reason the number is printed on the pipe instead of a wall thickness.
DR 11 means the outside diameter is eleven times the wall. On 2 inch IPS pipe with a 2.375 inch OD, that is a wall of about 0.216 inches. On 8 inch pipe the wall is much thicker, but the ratio, and therefore the pressure rating, is the same. SDR is the same idea with the ratio taken from a standard series of numbers; in the field the two terms get used interchangeably.
The DRs you will actually see
For PE4710 pipe the common pressure classes are DR 9 at 250 psi, DR 11 at 200 psi, DR 13.5 at 160 psi and DR 17 at 125 psi, at 73 degrees Fahrenheit. Ratings drop as temperature rises, and the manufacturer's listing on the print line is what governs, not a table on a website.
Why boring crews default to DR 11
Pressure is only half of what the wall is doing on a bore. Pullback puts the pipe in tension and the reamer and hole put it in bending, and a thicker wall takes both better, so DR 11 is the usual choice for conduit and service pipe getting pulled through a hole even where the operating pressure would allow something thinner. A thinner wall also collapses more easily under external load if the hole does not stay open.
What it does to weight and reel length
A lower DR is more plastic per foot, which means a heavier pull, a heavier reel and fewer feet of it on that reel. That is worth checking before the truck is loaded rather than after, and it is what the pipe weight calculator works out from OD and DR.