Regarding CFDI (Electronic invoice for Mexico) if a XML file has Document type = Transport (T), the value for tag Unit Value (ValorUnitario) can be zero or greater than zero.

At this time, the value appear as zero.

It would be desirable to include the option and choose in this case, if the value should be zeroed, or if should be the Unit Value of the goods transported.

Reference:

http://omawww.sat.gob.mx/tramitesyservicios/Paginas/documentos/Documentacion_tecnica.pdf

See page 78 of 137

ValorUnitario Validar Si el TipoDeComprobante es “I”, “E” o “N”, el valor registrado debe ser mayor que cero.