Custom Attribute - Secu - Searching for Encrypted Email

I need to know the possible values in the Secu attribute.

If I search Secu with a value of 1, I get a wackload of email so I am guessing that is unencrypted email.

If I search Secu with a value of 2, I get what I think is encrypted email. I see only the to/from/subject/date and no body. I get a very small number of emails.

Can someone confirm what the possible values are for the Secu attribute?

Thanks very much!