I found this formula in a website: Number of ways in which one or more objects can be selected out of $S_1$ alike objects of one kind, $S_2$ alike objects of second kind and $S_3$ alike objects of third kind $= (S_1 + 1)(S_2 + 1)(S_3 + 1) - 1$.
Where did they get this from? Does this use the concept of multinomial theorem? How is it derived?