Documentation for JIRA 4.4. Documentation for other versions of JIRA is available too.
Setting the Security Level on an issue restricts the access of that issue to only people who are a member of the chosen Security Level. If you are not a member of that Security Level then you cannot access that issue and it will not appear in any filters, queries or statistics.
The Security Level of an issue can be set either when the issue is being created or afterwards when the issue is being edited.
To be able to set the Security Level for an issue, your administator must add you to the appropriate Issue Security Level, and also grant you the 'Set Issue Security' permission for the appropriate project(s).
Note: