Your Atlassian account is your online identity that exists independently of the Atlassian products you use.
With an Atlassian account, you can use just one account to log in to any Atlassian products, such as Jira Work Management, Jira Software, Jira Service Management, Confluence, Bitbucket, Opsgenie, Statuspage, Trello. You don't need to manage separate accounts for each of these products - just one account to rule them all!
A portal-only customer account for Jira Service Management isn't an Atlassian account but is created and managed directly in Jira Service Management. You'll only have a customer account if you're logging into another company's Jira Service Management as a customer.