Whenever the Visual Fortran compiler has access to the use and definition of a subprogram during compilation, it may choose to inline the subprogram. Using statement functions and internal subprograms maximizes the number of subprogram references that will be inlined, especially when multiple source files are compiled together at optimization level /optimize:4 or /optimize:5 (or an appropriate /inline keyword was specified).
For more information, see Compile With Appropriate Options and Multiple Source Files.