A function declared in a mql5 program with the export postmodifier can be used in another mql5 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 EX5 functions exported by this ex5 file. Only function with such a modifier are accessible ("visible") from other mql5 programs.
The library property tells the compiler that the EX5-file will be a library, and the compiler will show it in the header of EX5.
All functions that are planned as exportable ones must be marked with the export modifier.