This is used to access and run a program.
Whether that program will return to the calling program depends on the called program: if the called program uses Return or runs to the end of the program, after the called program executes it will return to the calling program. If a Stop is called in the called program, it will not return to the calling program.
If prgmHIBYE is executed, the screen will display:
prgmHI was executed during prgmHIBYE, then once that was finished, the program continued.