Here is how the card number is formed :

37aa - bbcccc - cdeef

Two first digits : 37 = American Express
(Visa cards start with 4, Mastercards with 5, etc.)

aa = base currency code or card type
Some examples:
12, 15, etc. = US$ charge cards
42 = British pounds , 45 = US$ (International Dollar cards)
50 = Deutsche Mark (now Euro)
49 = French Francs (now Euro)
46 = some Optima credit cards
etc. etc.

bb + ccccc = account type and number
bb : 88 for Centurion cards (or old Platinum accounts).
Other Platinum or Gold cards start with '8'
bb starts with '9' for corporate cards
etc. etc.

d = card sequence number
Numbered from 1. If you loose your card, the new one will be issued with 2, then 3 etc.

ee = supplementary card sequence number.
00 : main card
01 : 1st supplementary card
then 02, 03, etc.

f = check number
To check the formal correctness of the card number