QW-403.9 For single-pass or multipass welding in
which any pass is greater than 1⁄2 in. (13 mm) thick, an
increase in base metal thickness beyond 1.1 times that of
the qualification test coupon.
What does this clause mean? What happens if a pass of thickness >13 mm is made?

Clause is very clear. Deposited weld metal thickness t more than 13 mm is possible if welding is done by SAW using 5 mm filler. In such case qualified t becomes 1.1 times as stated in clause. In any other process, it is not possible unless it is done intentionally.