In most cases, you would only be concerned with the last 3 digits, i.e. 644. The first digit is the attribute bit. It can also have a value of 4, 2 or 1 just as the other bits. 4 sets the userid, 2 sets groupid and 1 sets the sticky bit. When you set the groupid, that means execute or open the file as the owner's group. The same is true for set userid. It will treat everybody as if they were the owner.