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Mentions (often known as @mentions) are a useful way of drawing someone's attention to a page or comment, or assigning a task to them.
When you mention a user, they'll receive a notification by email and in their workbox; if you mention them in a task, the task is assigned to them and appears in their tasks list.

Only licensed users (YMEUACJiraServiceDeskUsers /YMEUACJiraYMEPAServiceDeskUsers/ YMEUACConfluenceLicensedUsers) are able to use mentions.  

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If you want to inform someone about an issue, but don’t feel like they need ongoing notifications, @mentions are a perfect solution.
You could also reassign a ticket to a colleague for consultation, but that’s a fairly heavyweight operation to just ask a question.
By using @mentions, you can quickly direct conversation in comments to get needed feedback and keep you moving forward.
That way the issue always stays assigned to you and on your radar.

The following users can being mentioned in Jira:

• Reporter
• Commenter 
• AsigneeYMEUACJiraServiceDeskUsers / YMEUACJiraYMEPAServicedeskUsers
• Jira Service Management agents

Participants of a ticket which are only Jira Servicedesk portal customers cannot being mentioned, unless they have left a comment in the ticket, then they will  become a commenter on the ticket.

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Mentions in Confluence

Licensed Users (YMEUACConfluenceLicensedUsers) in Confluence can being mentioned when they are having read/write or write authorization. 
Do not use a mention on a Confluence page for creation of documents because expired, removed or inactive users do not remain visible. So always mention full names.
Following group is only having read access and cannot being mentioned in a page.

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