For most players that club will be the driver.
In this instance, a driver is being used to measure the player’s 2 club length lateral relief area outside a red penalty area.
In this case, the back-with-the-line relief area is one club length either side of and behind the point he has chosen on the line from the hole through where his ball was unplayable in the bunker. He is estimating the size of his relief area without measuring, then dropping well within 1 club length of that point.
And when a player measures with a shorter club, it is still the longest club in their bag, excluding the putter, that defines the relief area.
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