any invoice that is issued, submitted and received in a structured electronic form such that it can be processed automatically and electronically; or
any equivalent request for payment that satisfies the above-mentioned conditions; or
any document or message that amends the initial invoice and refers to this invoice in a systematic and unambiguous manner, and satisfies the above-mentioned conditions, i.e.:
an XML file; or
a file containing XML markup; and
not simply a PDF, Word or another non-structured document which does not contain all the features of the electronic invoice (e.g. separate standardised attributes that can be read automatically by a computer).