What do linear rail codes mean?

In looking at 3D printer motions systems, I've seen linear rails with codes like MGN15H and MGN9C. What do the letters and numbers mean?

MG stands for minature guideway.

N signifies narrow, whereas W means wide.

So, there is MGN and MGW. You may also see BMN which is equivalent to MGN

The numbers, i.e the 9 and 15, refer to the track width. The widths are: 5 mm; 7 mm; 9 mm; 12 mm, and; 15 mm.

The final letter, i.e. C and H, refer to the length of the block, where C is the shorter version, whereas H denotes the longer elongated block which can distribute the load better (as it contains more bearings). These two lengths of the block can be seen compared, in the image below, which shows (from left to right) track widths of 15, 12, 9 and 7 mm, respectively:

Source

Note that there is an error in the diagram for the 7 mm rails - the lengths of the blocks (22.5 and 30.8 mm) have been, obviously, reversed.

A detailed description of the product code is shown below:

Source: Hiwin

The other features of the rails may need to be considered, such as their profile (square or round) and the bearings (cantilever or saddle), amongst other things:

Your choice includes the profile of the rails, commonly square or round. It involves the kind of bearings, either cantilever or saddle slides, plus the stroke, load, speed, duty cycle, mounting area, and of course the mounting orientation, simply because the saddle could potentially move horizontally, vertically, along a wall mount, or even be inverted.

Source: What is a Linear Rail?