Here are the definitions that I pulled from page 126 of the User Guide:
T -- The interface is a member of a VLAN. All packets forwarded by the interface are tagged. The packets contain VLAN information.
U -- The interface is a VLAN member. Packets forwarded by the interface are untagged.
F -- The interface is denied membership to a VLAN.
Blank -- The interface is not a VLAN member. Packets associated with the interface are not forwarded.