Subscriber Configuration Low/HighUp/DownlinkPriority

What are the “Low Priority Uplink”, “Low Downlink Priority”,
“High Uplink Priority” and “High Downlink Priority” items that are
listed on the AP Subscriber Configuration page? I’m guessing “Low Priority
Uplink” is really supposed to be “Low Uplink Priority”. I don’t find any
documentation on these items anywhere.