What feature distinguishes a union from a coupling in plumbing fittings?

Unions are distinctive in that they allow for disconnection, making them particularly useful in plumbing systems where regular maintenance or repairs may be necessary. This feature is essential because it enables plumbers to easily take apart pipe connections without needing to cut or damage the piping. Unlike couplings, which are typically permanent connections that require the entire section of pipe to be removed or cut if repairs are needed, unions facilitate easy disassembly and reassembly. This flexibility is crucial in many plumbing situations, especially in areas where equipment needs to be serviced or replaced frequently, ensuring that the system can be maintained efficiently without extensive labor.

The other characteristics associated with unions, such as being welded, having a smaller diameter, or being used exclusively for gas, do not accurately describe the functionality or purpose that differentiates unions from couplings. Unions can be used in a variety of applications beyond gas and are not limited by connection methods like welding or size.