Question of Invoice UBL (TaxTotal)

[Elena Pastor <epastorfayos@gmail.com>]

Hello,
I'm working on a project of a format electronic billing UBL (UBL Invoice) in Spain, and I need information to do so. 
I need information on how to include Withhelds Tax (Impuestos Retenidos) and Equivalent Surcharge (Recargo Equivalente), tax codes of spain, ...
All what is needed to properly mount Invoice.TaxTotal ...
Where can I find examples of UBL invoice files? And examples of Spanish invoice?

Can you give me any information about some link or forum where I can go to make questions about how to build and calculate the UBL Invoice document?
Is there an online tool where you can validate files of UBL Invoice of Spain?

Any help I would be helpful. Thank you very much in advance.

Regards,
  E. Pastor