4.11.5.4 M99: Return from Subprogram
M99 command should be programmed in the last block of external/internal subprogram to indicate the end of external/internal subprogram. After M99 command in the external/internal subprogram has been read, the cursor will return to the main program and continue to run the remaining blocks. If M99 command is programmed at the end of the main program instead of M02/M03, the program will repeat executing over and over again.