If you have a contact in your phone who has multiple emails or phone numbers, say for work, home, and cell phone, it's easy to lose track of which number bubbles up to the top when you search for it. How-To Geek points out that you can set up a default number that simplifies this.
The process here is a bit convoluted. If you have multiple contact methods listed, the first time you tap a blue icon in the new contacts field in iOS 10, whether it's the messages, call, or email icon, iOS will ask you which one you want to use. The one you select becomes the default.
When you pick one, it also initiates a call, text, or email, so be ready to cancel it out if you're just trying to set the default. Once the default is selected, it will be the one that autofills when you go to compose a new message. Changing this for something like an email address, where you might have several for one person, is extremely helpful, especially if iOS insists on using an iCloud email they never use as a default for someone. Head over to How-To Geek for a bit more info, including setting up Siri to work properly and your favourites list.