You can specify it on the Document or Line level.

Elements

Document level

Use the InvoiceNote element to add a note at the Document level:

Element (Path)	Cardinality	Description
Invoice Note (`Invoice/InvoiceNote`)	0..1	Specify your note.

Line level

Use the Note element to add a note to a line:

Name (Path)	Cardinality	Description
Invoice Line Note (`Invoice/InvoiceLine/Note`)	0..1	A textual note that gives unstructured information that is relevant to the invoice line.

Examples

Document level

<Invoice>
    <!-- code omitted for clarity -->
    <cbc:InvoiceNote>Please note our new address.</cbc:InvoiceNote>
    <!-- code omitted for clarity -->
</Invoice>

Line level

<Invoice>
    <!-- code omitted for clarity -->
    <cac:InvoiceLine>
        <!-- code omitted for clarity -->
        <cbc:Note>New article number 12345</cbc:Note>
        <!-- code omitted for clarity -->
    </cac:InvoiceLine>
</Invoice>