select target for incoming invoice number

configuration.py

@@ -9,6 +9,11 @@ from trytond.pool import PoolMeta
 from trytond.model import fields
 
 
+sel_number_target = [
+    ('reference', "in the 'Reference' field"),
+    ('number', "in the 'Number' field")]
+
+
 class Configuration(metaclass=PoolMeta):
     __name__ = 'document.incoming.configuration'
 
@@ -20,6 +25,19 @@ class Configuration(metaclass=PoolMeta):
         string='Accept other company',
         help='Accepts invoices created for a company other ' +
         'than the current one.')
+    number_target = fields.Selection(
+        string='Invoice number',
+        help='Destination for the invoice number of the supplier invoice.',
+        selection=sel_number_target)
+
+    @classmethod
+    def default_number_target(cls):
+        """ targetfield for incoming invoice number
+
+        Returns:
+            str: field 'reference'
+        """
+        return 'reference'
 
     @classmethod
     def default_create_supplier(cls):