Maybe a little clarification will help.
The welder welds the test coupon in the required position with GTAW for he root bead and the second layer. No backing is used. As a result of passing the test, the welder is qualified for welding joints in the position tested and any others permitted by the code for that test position. The welder is also qualified to used GTAW in production welds that utilize backing or those that do not utilize backing, i.e., open root joints. The thickness for which he is qualified is based on the thickness of he weld deposited with GTAW.
The test coupon was not welded completely. Several additional weld layers are needed to complete the test. The welder can complete the test with GTAW in which case he is only qualified for GTAW. As an option, the welder can elect to complete the test using a different welding process. In this case the welder elected to complete the weld using SMAW. Since the root and second pass are already in place, the remaining weld completed with SMAW is considered to be welded with backing. That is, the root and second layer served the function of backing for the subsequent weld deposited with SMAW. The welder is now qualified for SMAW if backing is used and the thickness is based on the thickness deposited using SMAW.
The welder is not qualified for GTAW with or without backing, but limited by the thickness deposited with that process. The welder is also qualified for welds deposited with SMAW, but he is limited to welds made with backing and limited by the thickness deposited with that process. He is limited by the base metal used, the F number of the filler metal/electrode, the position tested, and the weld type (groove versus fillet).
The welder can weld using only GTAW with or without backing. He can weld with SMAW, but only if backing is utilized, or he can use a combination of both GTAW (with or without backing) and SMAW, but only if there is a backing weld deposited by GTAW (remember, he is using both GTAW and SMAW on the same joint.
Hope this helps.
Best regards - Al