PC01 ticket designator has been turning up in public searches on UA.COM for a while now, at least since end of December. Seems to be bookable by the general public through UA.COM.

Does not seem to be published in GDS, although the corresponding fare without PC01 designator does. Differences are very minimal, last I checked there was no difference in price either. I'm not sure what the point of these designators is, maybe they serve some statistical purpose?