Many modern guitars have two-way rods that adjust in both directions. On those type, the nut is not removable. Single action rods generally have a removable nut. If loosening the rod does not meet any resistance, you probably have a one-way rod. If a truss rod is hard to turn, I remove the nut and lubricate the threads and the bearing surface. That is not possible with a two-way rod.
There are two basic designs for a two-way rod. On one, the rod itself turns when you adjust, creating a push/pull on a second rod or beam on top of the adjusting rod. This push/pull is accomplished by using left-hand threads on one end, or by using a different thread pitch. This rod cannot be completely unscrewed, because it is restricted by the second rod. Another design features a captured nut fixing it to the second rod, which creates the push/pull action. Capturing the nut means it cannot be removed.
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