How does Android contact linking work?

I'd like to understand how contact linking in Android works - or to be specific I'd like to understand it pretty deeply, **almost from an algorithmic point of view**. What I've observed on my phone is that some contacts were linked automatically, some were not (even though when you go to manual linking screen Android will suggest pretty good matches) and I'm confused about this. So my specific questions are: 1. What is the trigger for automatic contact linking? When does it occur? 2. Say that the answer to 1) is "when new contact is added, either manually or via account sync". So when this new contact is added what will happen? Will Android go through all existing contacts and try to find out if the new contact is just a linked contact? 3. When will Android recognize the new contact as a linked contact? When names match? Phone numbers? Emails? Some combination of those? Any insight would help clearing this up for me, thanks.