The miscellaneous charge order is a traditional paper voucher or electronic record issued by an airline or travel agent to collect funds for services outside of basic airfare. It handles transactions for excess baggage fees, cabin upgrades, pet transport, or refundable balances. While largely replaced by electronic miscellaneous documents in standard booking systems, veteran ticketing agents still use the term to describe any supplemental fee transaction.
The passenger has three overweight bags, so you'll need to issue an MCO before they can drop them at the belt.
If you hear an older gate agent ask for an MCO, they just mean they need to charge the passenger a fee using whatever the current electronic system is.
Related terms
Ticket designator
An alphanumeric code on an airline ticket indicating a specific fare type or discount.
Back to back ticket
A ticketing strategy using overlapping round-trip itineraries to bypass airline pricing rules.
Cross-border ticket
An airline itinerary booked from a different country to exploit regional pricing differences.