The Atlassian Account is mostly connected to your email address. This account is used for all Atlassian Cloud products. The Atlassian account is your online Atlassian identity that exists independently of Atlassian products.
Other authorities can add your account to their instance to collaborate.
The Atlassian Account is used in the complete Atlassian eco system which includes Jira Work Management, Jira Software, Jira Service Management, Confluence, Bitbucket, Opsgenie, Statuspage and Trello.
An Atlassian account is like a Google account. When you log into your Gmail account, you can also log in to YouTube, Google Docs, and Google Drive with the same ac. When you log into Jira with your Atlassian account, you can also log in to any other Atlassian product like Trello, Bitbucket, and others.