Shield Security plugin is composed of several parts (modules). Hack Guard is one of its modules.
More information about Hack Guard module can be found here.
The Hack Guard is composed of various components (features). The Unrecognised File Scanner is one of its features.
How does the Unrecognised File Scanner work?
The Unrecognised File Scanner identifies and eliminates all files that aren't on the WordPress Core File list. (Core files are the WordPress installation files)
More information about Unrecognised File Scanner can be found here.
What are the Unrecognised files?
Unrecognised files are the files that are not part of your WordPress core installation.
In other words, all non-standard files stored inside the WordPress core installation are identified as unrecognised.
What Unrecognised files should be removed?
The Unrecognised File Scanner will detect Unrecognised files and you decide whether to remove or to keep them.
Files that you want to keep can be excluded from the future scans.
More information on how to exclude Unrecognised files can be found here.
If you are still unsure which uncategorised files you should keep or remove, please talk to your web host and they'll help you determine what's best for your site.
For further reading on the Unrecognised File Scanner read our blog article here.
Note: You can now set the scan frequency by using Daily Scan Frequency feature. For more information on this, please follow the link here.