Invoice lines
Invoices are mostly a sum of a invoice lines (see How an invoice is calculated), with various information for each of them, and invoice lines are usually(*) an aggregation of rated transactions, that are the rated items ready to be invoiced.
(*) Invoice lines are usually an aggregation of rated transactions
This is the case for all invoices generated automatically (billing run, generate invoice), but when you create a manual invoice, you directly create invoice lines, without using the catalog and rating.
For more on invoice line generation, check Invoice line and invoice line generation
Invoice lines on Portal
Invoice line are visible directly on the Invoice page.

On the example above the invoice line is visible on the second section of the invoice page, in the DETAILED tab.
You can also unfold an invoice line to list rated transactions used to generate it.
It is also possible to Expand all invoice lines and add filters (:filter: button).