What is original Atlassian site URL and why is it required while setting up a cross site connection

What is original Atlassian site URL? Why it is required to setup cross site connection?

Original Atlassian site URL is like ( https://example.atlassian.net )
Revyz uses Atlassian APIs to back up and restore data. These APIs rely on the original base URL of your site.
When setting up a cross-site connection, you must enter the original site URL to ensure a seamless connection and data transfer.

Where do you get original Atlassian site URL?