⟩ What is need of START 0? In steed of can we use anyother numerics? if we use what will happen?
START is an assembler directive which has an optional
operand, (0 in your example). This operand acts as the
starting address of the program e.g. PROGNAME START X'3E0'
tells the linkage editor that this program is to be loaded
into main storage at address '3E0' in hex. This explanation
answers all your questions.