There are basically 3 different types of rack screws. 10-32 and 12-24 are used by different manufacturers in the United States. It really depends on the manufacturer. Some manufacturers use all 12-24, others use all 10-32, and some use a mixture of both.
The M6 (which stands for 6MM) is used in most of the world, outside the U.S. Even thogh the 6MM is non-US, there are some M6 racks brought into the US. There is also a 5M (5MM) that is occasionally used outside the United States.
Finding the Right Screw
The screws required depend on the rack (wall mount, floor mount, relay rack etc.) that the patch panel, shelf, blank, cable management bar or other panel item will be screwed into.
To figure out what type of screw you need for a rack you currently have, take out an existing screw and hold it up to a ruler. If you count 8 threads in 1/4" (32 threads per inch) it is a 10-32 screw (which is smaller in diameter than the 12-24 or M6 screw.) A 12-24 screw will have 6 threads in 1/4" (24 threads per inch.) A 6MM looks similar to a 12-24, however, it will have 25.4 threads per inch (or 1mm per thread, known as thread pitch.)
All of the shelves we sell come with both 12-24 and 10-32 screws. This allows you to install the shelf in either format rack.
Our swing out chassis and cabinets use 10-32 screws. We also have 4ft and 7ft two and four post racks that use 12-24 screws.