When Sender field is selected on a single item field, the field value found in the learning document (or in the document definition learned in Optimize) is saved in the document definition. All documents associated with this document definition will have this value assigned to that field.
This makes the value for that field static, though it can be changed in Verify.
You use this setting when:
A field value is always the same for a particular sender
You need a field value that does not appear on the document
Currency fields and bank account fields contain this setting by default.
Example
If one of your senders always has the same tax rate, you can select Sender field for the tax field. This ensures that the same value is always used for the tax field, and it increases the interpretation rate for documents from that sender.
Interpret still searches for the field in the position where it was learned.
If the field is found, the field status is green/Complete.
If the field is not found, the field status is yellow.
Regardless of whether the field was found or not, the value is always set to the static value specified in the document definition.
Tip: By using the No interpretation setting in combination with Sender field, the field will always be set to Complete in Verify, regardless of whether it was found or not, or whether it has validation errors. See Setting Sender fields to Complete automatically.