Clarification on Customer Model Changes by Charlie

From an email I just received:

Customer data may not have complete information. Your customer model data may include some, all, or none of these fields:

Email
Address
Name
Phone
Zip
Etc.

I was under the impression email was always required of Shopify customers, and it was mandatory that each customer would have a unique email. Is this still the case? What does Shopify use as the unique customer identifier now? How can one prevent duplicate customers? 

Also, if customers are no longer always attached to an order, does this mean that when a customer completes an order on stores that do not have customer accounts enabled that their information may not be saved? I don't quite get this change, unless it's perhaps to make way for manual order entry.

Any further clarification Shopify people can provide would be great, thank you!