The 4-digit number is 18E7 or 1818. Remember these are hexadecimal digits. Hence, a 4-digit number.

18E7 -> 18 is code E7 is subcode
1818 -> 18 is code 18 is subcode

So, the format is CCSS where 'CC' is the custom code in hexadecimal and 'E7' or '18' is the subcode in hexadecimal.

What you may or may not know is that many vendors supply data in either hexadecimal ore decimal format. With a number like 18, it's hard to tell. However, when 'E7' is presented, you immediately suspect you are working with digits 0-9 and A-F which are known a base-16 or hexadecimal values.