A connection can contain threads without sealing on those threads. On a compression fitting, the nut pushes an olive against the pipe and fitting. A union pulls mating surfaces together. Some radiator tails seal on a thread; others use a washer or O-ring.

The useful question is “which surfaces and materials form this seal?” Identify the actual product before specifying tape, compound or a replacement part. The examples explain an installer’s work; they are not instructions to loosen a joint on a live circuit.