Hi jmelwak,
Credits are manual credits/bonuses issued by the casino, whereas, coupons are automatic bonuses that are credited to a player account when they deposit using a bonus code. In your situation, the player did not use a bonus code when he/she first deposited so the casino manager manually credited the player account with the welcome deposit bonus. Basically, credits are promotional cash credits. Let me know if you need any further explanation(s).
Cheers,
Ryan