Also called a NOC (not otherwise classified) code: a placeholder used when no specific code describes the item.
J3490, J3590, J9999 and A9999 are the common drug examples. They shift adjudication from automatic to manual, so every miscellaneous claim needs a description, an NDC, a quantity and usually an invoice.
Payers routinely audit miscellaneous codes because they are the easiest place for pricing errors to hide.