3.13.2 Backing. Prequalified CJP groove welds made
from one side only, except as allowed for tubular structures,
shall have steel backing. Other backing, such as
listed in 5.10, may be used, if qualified in conformance
with Clause 4.

The above clause is appropriate for only welds with backing (welded from one side)

B-U2-GF is a joint configuration that requires welding from both sides.

B-U2-GF does not distinguish plate or pipe

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