Technically correct would be to call the keys in Kleopatra for S/MIME X509 Certificates, which can be used in the Cryptographic Message Syntax to encrypt. For example MIME Messages in the S/MIME Standard.
This is what Kleopatra did in the past. But users are used to "S/MIME + OpenPGP" at most. Everything else is too technical for the masses.
Feedback I get from institutional users is that their users are confused.
So in certificate details we might still talk about X.509 but in all places where end users and not admins should come into contact with keys this should be changed.
I'm closing this task after making some small changes but I want to document this decision for the future.