Difference between 'Rules' and Smart Mailbox'

Smart Mailboxes just displays messages that meet the given criteria. It doesn't move them, alter them, forward them, or do anything else to them. If you just want to see all the emails from Aunt Mae, then you can create a Smart Mailbox that does that.

If you want to move all the messages from Aunt Mae into a new mailbox, or mark them in blue, you do that with a Rule.

Rules only act automatically on messages that enter the inbox. Otherwise, you have to manually run the rule.

Smart Mailboxes just display things that match a criteria. Rules can perform actions on the messages that match a criteria.

Use Smart Mailboxes in Mail on Mac - Apple Support

Thanks. I was pondering the best way to explain it. I think you explained it well.