To CFDI 3.3 the SAT provide the new catalog that need be used to
"RegimenFiscal" node used by the company that emit the documents.
Definition:
Atributo requerido para incorporar la clave del régimen del
contribuyente emisor al que aplicará el efecto fiscal de este
comprobante.
And the attribute need the code from the catalog, and validate that it exist.
In CFDI 3.2 the attribute was required but free. Then the code also
could be used in CFDI 3.2.
Like fiscal position have not field to save the code, is used the next
structure
`code - name`
And when is used this element in the XML generation take only the first
three characters.
`company_id.partner_id.property_account_position_id[:3]`
Luis Torres