What is this white rectangle painted on runway 23 at KCRW?
Seems to be touchdown zone markings for assault landing practice by C-17's. For assault landings, the standard is to touch down in the first 500' of the Landing Zone (LZ), and to go around if your mains aren't on the ground by the 500' point. On a dirt LZ, there will typically be panels set out to mark the threshold, the 500' point, and the far threshold -- that way the combat control team which is marking the LZ doesn't have to put out lots of panels, but there are enough for the crew to get what they need. The 500' markings on the main runway would give enough information to the crew to simulate landing on an LZ, even though the actual runway is far longer &amp; wider.

This frame below shows examples of C-130's practicing assault landings, and you can see orange panels at the threshold and 500' points in them:


Source: YouTube

Some places, KLRF (Little Rock Air Force Base) for example, had an actual 3500' long paved strip for crews to use for assault landing practice, with permanent markings at the threshold and 500' points (2013 imagery shown below):


Source: Google Earth

Most airports don't have the budget to create a separate runway for this specialized requirement, so some sort of markings on the main runway are used instead.