why we use only 0 (not other digits) after return in main function

Answer 1

The C++ standard forbids the use of a void return type for the main function
http://www.stroustrup.com/bs_faq2.html#void-main

So the main function must return a value. It can be any value. Most compilers use the return value of main to denote its "Exit Status", where 0 represents a successful exit status, and anything else (usually -1) represents an unsuccessful exit status.

Answer 2

if you created a function of integer type then you can return any value but "it should be in integer type "
so if you want to
return 5
then you do so,
->because function "int main() " or any "int abc()"
tells you to return a integer not tells you to return 0
only.
example :-int main(){
int a=1,b=4;
return a+b;
}
here you not have to return 0
because function already return an integer value, which is a+b