E-Commerce Configuration Guide : Site Configuration Settings : Checkout Settings : CouponCodeField
 
CouponCodeField
Key value:
srcecode or mktgcode
Description:
This key is used with specials. It designates whether to use marketing codes (mktgcode) or source codes (srcecode) to apply the specials. Specials are enabled using EnableCheckoutCoupons.
See also:
EnableCheckoutCoupons