TC-U4A is a joint designation, a prequalified complete joint penetration groove weld, smaw process. This joint preperation can be found in the structural welding quality handbook or AWS D1.1. It`s a "T" joint that is beveled on one side. Hope this helps you, Have a good one!

The "C" also designates that it can be a corner joint.

The joint described requires a backing bar, If it was written as TC-U4b then it would not have a backing bar, but would be back gouged instead.

This joint, like said in earlier replies, is a prequalified single-bevel full penatration groove "T" or Corner joint. It can have a 1/4" root opening if the groove angle is 45 degrees, and 3/8" root opening if the groove angle is 30 degrees. If the 45 degree bevel is used, it is permissible to weld in all positions and the 30 degree is limited to flat, vertical, and overhead positions only. See Figure 3.4 on Page 92 in the AWS D1.1:2002 code book for all the tolerances and specific details about this weld designation.
John Wright