Each application has a permission setting for "people in this group can use the system". The groups, by default, are separate, so you'd have groups like "Jira-software-users", "Confluence users" and "Bitbucket-users", and in the applications, Jira software will say "let people in Jira-software-users can log in" and Bitbucket and Confluence will do the same for their user access.
So you can have 25 users for Jira, 10 for Confluence and 10 for Bitbucket, so 45 users in total, none of whom have access to anything but their own applications. A more normal situation would be to have a developer in all three groups, and hence have access to all three.
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