Reference field and prefix. Each document carries the Mercura reference: the prefix (MERCURA-REQ- by default) and the request number. The reference goes into AssignmentReference (at most 18 characters, so the prefix takes at most 12) or one of your own fields. Sales quotations have no AssignmentReference and carry the reference in the customer reference (PurchaseOrderByCustomer).
Duplicate check. With Look a document up before sending it again, Mercura searches SAP for a document with the reference before it sends the document again, for example after a time-out, and takes its number instead of creating a second one. This needs the reference field to be filterable in your system; check that on your test system first.
Units. Mercura sends each line's unit as an SAP unit of at most three characters (e.g. Stück → ST). Under Units you map Mercura units to your SAP units; a line without a unit gets the default unit.
Further fields. Further fields of the header and the lines, such as ShippingCondition or your own YY1_… and ZZ1_… fields. Each takes a value from Mercura: the request number or name, the project name or object number, the customer number, the responsible user's e-mail, the customer's order number, order date, requested delivery date or quotation number, an order field, a custom field of the request, a fixed value, and for lines also the position number, short or long text, article number or an article attribute. Fields the connection writes itself cannot be mapped.
Link to the document. An HTTPS link that opens the document in SAP Fiori, with {erp_offer_id} where the number goes, e.g. https://s4.example.com/sap/bc/ui2/flp#SalesOrder-display?SalesOrder={erp_offer_id}. The link opens SAP in the user's browser, so it needs their network access and SAP login. Without a link, Mercura shows the number as text.