The update contract functionality only syncs to contracts previously created from the current quote. The sync operation overwrites existing contract header and line item data with current quote values. And custom field mapping only occurs when API names match exactly between Quote and Contract objects. If the contract was originally created using only selected line items, performing an update operation copies all existing cart line items into the contract along with any changes in the quantity or pricing.

For example, suppose the cart contains 10 line items, each with a quantity of 50, and you create a contract using only the first five line items. Later, you update the quantities of the first three line items to 100 and perform an update operation. As a result, the contract now includes all 10 line items instead of the original five. The first three line items have a quantity of 100, while the remaining items retain a quantity of 50.