On the debit side? Sure, I can, and I'll try to be brief. Cut me off if I'm not brief enough.
Priority routing refers to a practice where, if you had an Interac card today, and let's say Maestro was added to the card, the instructions from the issuers would be to route this through Maestro as opposed to an Interac transaction. The card brands have taken different approaches on this. Visa, more recently, has said that they're going to allow consumers to choose. MasterCard is asking the banks to indicate where they would like their customers' transactions routed through. So they've taken different approaches, as they will do occasionally.