Invoices are Payment Requests signed by their requestor.
When you click on a bitcoin: link, a URL is passed to electrum:
electrum "bitcoin:1KLxqw4MA5NkG6YP1N4S14akDFCP1vQrKu?amount=1.0&r=https%3A%2F%2Fbitpay.com%2Fi%2FXxaGtEpRSqckRnhsjZwtrA"This opens the send tab with the payment request:
The green color in the "Pay To" field means that the payment request was signed by bitpay.com's certificate, and that Electrum verified the chain of signatures.
Note that the "send" tab contains a list of invoices and their status.