Several websites, one hosting plan.
THE STRUCTURE
Each addon domain has its own directory and usually its own database. They are separate sites sharing an account.
WHAT THIS MEANS FOR BACKUPS
A full account backup covers all of them in one archive, which is convenient and large.
Per-site backups let you restore one without affecting the others, which is usually what you actually want.
THE PRACTICAL ARRANGEMENT
Per-site backups: each site's files and its database, separately, so a problem with one is fixed without touching the others.
Plus a periodic full account backup for everything else: email, cron jobs, account settings.
KNOWING WHICH DATABASE BELONGS TO WHICH SITE
Check each site's configuration file. With prefixed names across several sites, this becomes genuinely confusing.
Record it. During an incident you do not want to be working it out.
RESTORING ONE SITE
Restore that site's directory and its database. Leave the rest alone.
STORAGE
Several sites means several backups. Watch the storage cost and set retention per site.
