Per-Line Descriptions on Contract Items

Contract line items can now carry their own invoice description:

• POST /contracts accepts an optional description on each item in items. It overrides the product's description on every invoice line generated for that item, so the same product can read differently on different contracts. Omit it, or send a blank string, to keep using the product description.
• Contract item responses now include description.

Applies to V3 and V4.