In both March & May at PO French Quarter we were told we could only trade CS credit at breakast to make a "continental breakfast"...the ladies at the cash registers had a card that showed exactly what was allowed. The examples were the following 3 snack credit items could be purchased using one counter service credit:
pastry+single serving fruit+beverage=1 counter service
or cereal+single serving yogurt+beverage=1 counter service
there's was one other version on her list, but don't remember what it was...maybe fruit+yogurt+beverage? And she did say you could mix & match on the acceptable items so could do cereal & fruit & beverage for example.
Other than the continental breakfast, they were telling people no on using snack items for counter service meal (a guy asked about taking a bunch of bags of individual chips & cookies to use up his counter service and was told they couldn't do it. She did suggest he take sandwich & chips with bottled beverage for the road.)