Rear axle sizing for wheel set, compatible with both 142mm thru axle and 135mm quick release?

DT Swiss often includes different kind of end caps with their hubs or wheels, I know mine did. The consumer can just install the correct end caps before installing the wheels. The free hub is not relevant in that case as the it's the end caps that make it compatible with either 135mm QR or 142mm TA.

Like Luuklag commented, check with the seller if the wheel set actually includes the end caps you need regardless of what is listed on their website (shipping back the wrong wheelset could come with a hefty postage charge).

I'm extremely wary of what online retailers say about their products. I also would not assume that all 12mm TA / 142mm wide hubs are convertible to 135mm with some end caps. If the hubs are wider than 135mm end caps cannot make them narrower. (A friend of mine recently made this mistake with some Roval wheels.)

The first thing to do us look at the specs on the DT Swiss page. DT Swiss does not explicitly state 135mm/QR compatibility. What is does say is 'Built in Dimension' is 142mm which heavily implies they wont fit in a 135mm frame, but then a '5mm endcap kit' is included which rather implies they will.

Next step - find the Spline Wheel Manual via the support page. This explicitly gives instructions for converting both front and rear wheels with end caps so I'd say you were good.

If you really, really want to be sure, contact DT Swiss directly.

135 QR and 142x12 thru axle are two wholly different standards with no intrinsic cross compatibility.

They both locate the cassette in the same place relative to the frame centerline, or in other words they give the same chainline. 142x12 is essentially the iteration of a thru axle rear end that didn't change anything else about the parts that would play nice on the rest of the bike, coming out of the long period where 135 was the only game in town for a disc bike.

All that said, it's extremely common for aftermarket 142x12 wheelsets and hubs to have 135 QR adapter endcaps either included or available. These are all proprietary to the hubs they're made to go in. It's pretty common for them to be included and I believe that is the case for the DT wheel in question.

Whether the freehub body is XDR or XD or HG or Microspline or whatever is immaterial to the frame the hub is going in. All that matters is either endcap availability or native compatibility with the standard the frame is made to.