This dialog box serves to store the Public Key ID files of people who trust you and you want to trust. Call it from Security > IDs and Certificates > Trusted Identities using this tool:

Click Browse to locate and add a Public Key ID file provided to you. Typically, the extension of .p7b, .p7c or .cer is assigned to this file. To see the typical contents of such a file, go to Digital Identities Overview. You should also add the certificates of Time Stamp Authorities to use their services to verify the authenticity of dates and times in signatures. Add the certificate from any Certificate Authority (CA) you or your partners wish to use.

• Click Search to help you locate such files on Directory Servers maintained by Certificate Authorities.
• Click Details to see all available information inside the selected ID file.
• Click Export to send the selected file to a location in your system for reuse.
• Click Delete to remove the selected trusted ID file.
• Select a Trusted ID file from a Certificate Authority and click Set as a Trusted Root to always trust any document sent via this Authority.