File with a .M extension contains an implementation of a class for Objective-C programs – the @implementation directive, initialisation of required variables and function definitions, to which other source files may refer to. M files can also comprise of references to header files – H.

M files are used by programs written in the Objective-C language, usually utilised by applications for Apple systems – Mac OS and iOS. Both platforms have dedicated APIs – Application Programming Interface for Objective-C, Cocoa API for Mac OS and iOS API, which is used to communicate between the application and the system.