It means that the airfield is 'Unverified'. It gets this status whilst Airservices tries to establish reliable contact with the owner in order to keep the data maintained. If no contact is established or the owner is unwilling to maintain the data, the information is removed from the charts.
Thats what the U is for.
Thats why some ALA's have been removed.
Establishment of a data originator is a CASR 175 requirement. No data should be published without a recorded data originator.
Alpha