The Generalitat de Catalunya recommends including the file number in the “FileReference” element of the invoice (the same for all the detail lines of the same invoice). Facturae / Invoices / Invoice / Items / InvoiceLine / FileReference
Although these indications are not mandatory for local administrations, they are good practices that, if extended to the majority of Catalan public administrations, can make it easier for internal invoices to be sent to the corresponding departments of each administration. You can find more information on the file number in the following note from the Generalitat: Information Note File Number
As a supplier to the Generalitat, you will find more information at:
https://economia.gencat.cat/ca/ambits-actuacio/factura-electronica/
