DIN rail

A DIN rail is a metal rail of a standard type widely used for mounting circuit breakers and industrial control equipment inside equipment racks. These products are typically made from cold rolled carbon steel sheet with a zinc-plated or chromated bright surface finish. Although metallic, they are meant only for mechanical support, and are not used as a busbar to conduct electric current, although they may provide a chassis grounding connection.
The term derives from the original specifications published by Deutsches Institut für Normung (DIN) in Germany, which have since been adopted as European (EN) and international (IEC) standards. The original concept was developed and implemented in Germany in 1928, and was elaborated into the present standards in the 1950s.
Types.
There are three major types of DIN rail:
Top hat rail IEC/EN 60715.
This 35 mm wide rail is widely used to mount circuit breakers, relays, programmable logic controllers, motor controllers, and other electrical equipment. The EN 60715 standard specifies both a 7.5 mm (shown above) and a 15 mm deep version, which are officially designated
Some manufacturers catalogues also use the terms Top hat section / Type O / Type Omega (Ω).
The rail is known as the TS35 rail in the USA.
Module width.
The width of devices that are mounted on a 35 mm "top hat" DIN rail generally use "modules" as a width unit, one module being 17.5 mm wide. For example, a small device (e.g. a circuit breaker) may have a width of 1 module (17.5 mm wide), while a larger device may have a width of 4 modules (4 * 17.5 = 70 mm wide). Equipment enclosures also follow these module widths, so an enclosure with a DIN rail may have space for 20 modules, for example. Not all devices follow these module widths. 
Module widths are usually abbreviated as "M" (e.g. 4M = 4 modules) . Some manufacturers (including Mean Well) use "SU" (likely stands for "standard unit", e.g. 4SU = 4 modules) .
C section.
These rails are symmetrical within the tolerances given. There are four popular C section rails, C20, C30, C40 and C50. The number suffix corresponds to the overall vertical height of the rail.
G section.
G-type rail (according to EN 50035, BS 5825, DIN 46277-1).
G rail is generally used to hold heavier, higher-power components. It is mounted with the deeper side at the bottom, and equipment is hooked over the lip, then rotated until it clips into the shallower side.
Others.
In addition to the popular 35 mm × 7.5 mm top-hat rail (EN 50022, BS 5584, DIN 46277-3), several less widely used types of mounting rails have also been standardized: