LRX21R started going out as an OTA for Nexus 9 users on the LRX21Q build yesterday, but today Google has posted the complete factory image. You can get it here.
Hopefully this new R build will upgrade users currently stuck on LRX21L (the OTA to Q simply fails), though we don't know that for certain, as it's currently only rolling out in OTA form for those on the Q build. Other bug fixes and changes could be in here, too, though, like the odd issue with the Nexus 9's OK Google Everywhere not, you know, working.
The Q build brought significant performance improvements and stability fixes to the Nexus 9, in our experience, so if you haven't flashed Q yet and are still stuck on L, I'd definitely recommend giving R a whirl.