A function declared in a mql4 program with the export postmodifier can be used in another mql4 program. Such a function is called exportable, and it can be called from other programs after compilation.
This modifier orders the compiler to add the function into the table of EX4 functions exported by this ex4 file. Only function with such a modifier are accessible ("visible") from other mql4 programs.
The library property tells the compiler that the EX4-file will be a library, and the compiler will show it in the header of EX4.
All functions that are planned as exportable ones must be marked with the export modifier.
When compiling libraries in the strict mode, the export modifier should be added for each exported function, otherwise the function will not be accessible from outside.