You can set a security level to be the default option for issues if none are selected. This can be a useful feature for projects with a high-security requirement to prevent users (with the Set Issue Security permission) from forgetting to assign a security level for their issues:

1. Browse to the Edit Issue Security Levels page
2. Click on the Default link for the security level you want to set as default

Once set as default, the security level will have Default next to its name. Now, when the user creates an issue and does not assign a security level, the default security level will be applied.