What does the 'Mark as default' option do in my 'Edit' option of a contact
I'm looking for documentation to back this up, but in my experimentation the "default" phone number for a contact is what's used with voice commands if you don't define a phone "type".

So, if I have three phone numbers for John Smith, if I just use the command Call John Smith the default phone number will be used. Otherwise, I need to use Call John Smith home or Call John Smith mobile.

It's also used if, when looking at a list of contacts, you tap the phone icon next to the contact's name. It will use the default phone number (if defined).