JIRA (and for that matter most Atlassian applications) have a simple way to count users against licences. If you can log in as a user, you count towards the licence.
However, that model does not work for Service Desks. You might be supporting 1,000 people in the organisation, 200 of whom are developers, but you only have 5 people on the helpdesk. Licencing 800 people for full JIRA access that they simply won't use is not fair (or cheap). So Service Desk does "Agents". For the example I've just given, you would need:
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