Modifier is a term used in Payments Manager+ to modify the price of an item based on the specification.
Item A has an option to be upgraded to Medium or Large. Medium adds $2 to the original price. Large adds $3 to the original price. Merchant selects item A with base price of $5 and upgrades it to medium. On the invoice, Item A will become $7 because of the chosen modifier.
Modifier is best used on the enhanced version of the Invoices module.