This policy setting allows you to turn off the indexing of the body and attachments of clear-text signed messages. The sender, ...

This policy setting allows you to turn off the indexing of the body and attachments of clear-text signed messages.  The sender, subject line, and date will continue to be indexed and searchable. 

If you enable this policy setting, indexing of clear-text signed messages will be turned off.

If you disable or do not configure this policy setting, clear-text signed messages will be indexed and searchable.