By definition the To recipients are required attendees, the CC recipients
are optional attendees, and the BCC recipients are resources. So any mailbox
placed in To or CC is not a resource, and is either a required or optional
attendee.
Depending on the setup of that mailbox, and whether anyone actually logs
into it, any possible autoaccept behavior will vary, but in any case it
won't be a resource mailbox any longer if a To or CC.