Hi Barry - Welcome to the Atlassian Community!
Statuses can be added to a workflow at anytime by a Jira Administrator. I am pretty sure that "mitigated" is not one of the out of the box statuses that come with Jira. Therefore, as Nic said, someone on your team must have added that status.
Also, if the team has been using that workflow for awhile, then any member of that team should know what context it is used for in that particular workflow.
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