Ah, then this is the (Trytoin internal) ID.
Then we need an additional field “customer’s Blanket Order Number” which contains the number (or string) the customer gives for this Blanket Order, and which has to be printed on the documents, resp. transmitted in the EDI message.

Manage Blanket Orders (#10962) · Issues · Tryton / Tryton · GitLab
Following https://discuss.tryton.org/t/managing-blanket-orders-sales-and-purchase/4675 Add two modules for managing blanket orders: Purchase Blanket Order and Sale Blanket Order