It is possible to split the definition of a class  in two or more source files. Each source file contains a section of the class definition, and all parts are combined when the application is compiled. There are several situations when splitting a class definition is desirable:

When working on large projects, spreading a class over separate files allows multiple programmers to work on it simultaneously.
When working with automatically generated source, code can be added to the class without having to recreate the source file.