Three parts with three jobs: the raceway carries and protects the conductors, the fitting joins the raceway to the enclosure, and the box encloses the connection.
Parts that look interchangeable are not. Using a coupling where a connector belongs leaves a system that goes together and is not the system anyone specified.
The two that get mixed up are the connector and the coupling. A connector is what enters a box -- it is the transition from raceway into enclosure. A coupling joins one section of raceway to another section of raceway. Same family of parts, two different jobs, and each one has to be chosen for the actual system you are building.
Check yourself: You need to join two lengths of raceway to each other. Connector or coupling?
Answer: A coupling. A connector enters a box; a coupling joins raceway sections.
They are named for what they connect. The connector is the raceway-to-enclosure part and the coupling is the raceway-to-raceway part, so the job decides which one it is.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
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