What is the difference between the visa category being U (Unavailable) and mm/dd/yy (quota backlog)?

“U” or “Unavailable” means that there are no more immigrant visas available at all in that category for the month. If there is a date noted (i.e. 07-01-03), it is considered to be the cut-off date, and that means that there is a “quota backlog”. Only individuals who have a priority date earlier than the cut-off date may move forward with the permanent resident process.