I have purchased tickets directly from their website for my wife and my visit this June. When opening the page "Bill" it requests information; One request is "Document"; and the choices are
NIF
CIF
NIE
EUVAT
Other
Any idea what they want from me? My Passport #?

no, it would be helpful if you explained exactly where you see this stuff because I just looked at the website and made a purchase up to clicking the pagar button and no such information with those terms was ever requested. It just asked for your name, country, email and credit card number info. So what country did you say you were from and how are you trying to pay? That must be the difference

Those are Spanish ID card numbers of various types. NIE and NIF are foreign ID card numbers if you are a Spanish resident but were granted residency, CIF is a company's tax ID, EUVAT is a European VAT number.

Thanks. The page comes AFTER you pay.
I have learned that this 'detailed bill' is for people who needed a detail receipt, showing taxes, etc.