Thank you Jürgen.
I have found useful the sap note 114213, specially the paragraph which says:
"The default document type is no longer taken into account.
If there are several allowed document types, the document type that is entered in the first position for the relevant selection in the table T161A has priority."
So my question is: "Is it possible to rearrange the positions on that table in order to select another document type as the default one?"
Thank you all again.
Regards