ExpertFlyer and expert mode on UA.com both show the same availability for this flight now (J8 JN8 C8 D8 and the rest zero).

A few things may explain this:
1. They temporarily zeroed out the business buckets as they were adjusting inventory (most likely);
2. The data feed to expert mode on the web site was broken temporarily and fixed itself.
3. Someone really had just put all 8 remaining business seats on hold but then cancelled them.

As for which bucket to track...R is used for instrument-supported upgrades using GPUs or miles.

-RM