UBL Newbie -- invoices

[Manuzhai <manuzhai@gmail.com>]

Hello,

I am a small business owner from the Netherlands. I am in the web
development business (mostly selling services instead of products --
I'll get back to that), and I like (XML) standards, so I decided that
I wanted to rewrite my *very* simple invoicing system to produce UBL
Invoices and render them to PDF and/or XHTML. In doing this, I've got
some questions, though, and as I haven't been able to find much
documentation about using the format, I decided to ask my questions on
this list. Consider these newbie implementation questions.

1. Is there an easy-to-read list of available elements that I can use
in cac:Party, or maybe in cac:Party/cac:Address? In addition to the
default elements listed in the example documents in the UBL package, I
already found cbc:Telephone and cbc:ElectronicMail. Should these be
put into cac:Address, or one level up in cac:Party? Also, in the
Netherlands it is required to put a VAT number and a Chamber of
Commerce ID for the seller in the invoice. Finally, I would like to
advertise my website on the invoices. Is there a default element I can
use for this? I've looked through the .xsd's for cac and cbc, but I
wasn't able to find any elements.

2. Since I may have multiple projects for one client, is there a way I
can reference the project the current invoice is concerned with? I
used to display this with the BuyerParty information, but I'm not sure
where to put it now (or how I can describe it).

3. What are options for the cbc:InvoicedQuantity/@quantityUnitCode?
The example uses PKG; since I'm often talking about hours of
development, I have a few examples of 1.5 hours and I'm not sure
whether this is a valid use for this element.

4. In the InvoiceLines, Tax:Scheme/cac:TaxTypeCode is used. What are
valid values for this? Should I use my national translation for VAT,
"BTW", or stick to VAT? What are good values for cac:ID in
cac:TaxCategory.

5. In cac:PaymentTerms/cac:ReferenceEventCode, what are valid values,
and what does the "!" value used in the examples mean?

Now, all of this might be documented somewhere, but I wasn't able to
find it in the standard package and a quick Google search didn't
reveal many other resources. Any pointers to more human-readable docs
are appreciated. All in all, this seems a very nice, workable standard
(even for the non-standard use I'm putting it to).

Thanks in advance,

Manuzhai