From The Free dictionary by Farlex, a receptacle is defined as “A fitting connected to a power supply and equipped to receive a plug.” I like this definition it describes both halves of the receptacle. In the automotive industry, the back half of a receptacle has threading on the nut with leads that could possibly connect a power supply. The front half describes which kind of cordset is needed. Typically, receptacles are used in a control cabinet, where there is easy access and out of the movement of machinery. Inside a control cabinet is a power source and/or programmable logic controller (PLC) which a receptacle would be wired to in the configuration of the controller. Receptacles used on a control box normally have a tight seal to keep out moisture and dust.
When looking at a receptacle there are two ends with different kinds of threading. In the front of the receptacle has a connection for a cable to connect to the outside environment, cells, and machinery, to the control box. The different cables could have diameter widths of M8, M12, 7/8”, 1” and more. From the picture, we see the front side of the receptacle calls for the M12x1 which would use a M12 cable. The first number is always the diameter of the outer threads. The other end of the receptacle, ½”-14NPT, where the leads come out, has another diameter referred as to the mounting type. There are many different kinds of mounting: Metric, PG, NPT, front mount, back mount, panel mount, etc. The two mounts types being explained here are Metric and NPT.
As mentioned above, there are many types of mounting, but let’s get into more detail about metric mountings. A metric mount has threading is evenly spaced throughout the screw with no degree of incline. To explain a variation of a metric mount, I will use M16x1.5 as an example. 
If the number is broken down, M16 means the diameter of the receptacle is 16 mm from the farthest edges and it would take 1.5 turn to move the receptacle in 1 mm. To explain it again, to move the receptacle in 2 mm, turn the receptacle 3 full turns. When a receptacle has this type of threading, an o-ring will probably be needed to have a tight fit between the receptacle and the control box.
NPT, National pipe thread, is the US customary unit version of threading. Like the name implies, NPT threading is used on pipes, screws, and any product with threading containing inches. A variation of NPT threading on a receptacle is ½”-14 NPT. To break down ½”-14NPT, the major diameter is ½” wide, and the next number is the amount threads per inch. Based on the example, there would be 14 threads per inch. NPT threading is at angle which allows it to self-lock. These self-locking threads give a tight fit to keep out moisture and duct.
Receptacles available in the automation market have a variety of combinations on both ends of the receptacle.
Nice job Janet — I will be sure and share this on our blog too! ~Lynne (Mencom)