UBL is an XML syntax for many different business documents, among others invoices and credit notes. E-Invoice-EU currently only supports invoices.
The format is well documented by Peppol at https://docs.peppol.eu/poacc/billing/3.0/syntax/ubl-invoice/.
The current official documentation of UBL is available as an OASIS Standards Document. But it is not very helpful in the context of electronic invoices in the EU because the EU has additional requirements. It is therefore recommended to use the Peppol documentation.
Clicking on an element name opens the documentation for the element. If the name
of an element begins with cac:, it has subelements. If the name starts with
cbc: it is the name of a leaf element. It makes sense to get familiar with
this documentation because the Peppol UBL invoice structure is also the basis
for the internal invoice format of E-Invoice-EU.