Alternate/alternative email addresses

Where’s the best place to look for documentation on alternative emails?

If a user has an alternative email address, is it possible for him to choose which email address should be sent notifications? Or will they always go to both, or just to the primary address? What I have in mind is that people might register using their personal email account but later add a work email address in order to obtain automatic membership of a group. It would be a pain (and likely go against their reason for not registering with the work account) for email notifications to be sent to the work account in this scenario.

Does Discourse maybe spot when the primary email is bouncing and use the alternative one temporarily until the deliverability problem is resolved?

Is the alternative email mainly for allowing login from different third-party services (eg their Facebook and Twitter might have different validated email addresses) rather than anything to do with email notifications?

Is it possible to swap primary and alternative emails? I saw an old topic about this and I think the answer at that point was not directly/straightforwardly.

Is it possible to have more than one alternative email?

Thanks.