With invoice external URL you can share the invoice to a customer via a link.
With this link, they can see & print the invoice without they having to login into the system.
The link can also be sent in Email & SMS notification. To send url in Email/SMS notification add it in notification template using the url tag.
Steps
Go To Sell > All Sales (Link)
Click Actions (of the invoice)
Scroll down and find invoice ur
Click it and it will show you the Url of link
Alternatively, you can include invoice link in printed and digital invoices on invoice layout settings