I don't know the best practice but this ow I have setup my UTM tags for my eCommerce site

For New Account creation

 - Campaign Source : website 
 - Campaign Medium : email 
 - Campaign Name : new_account_email 
 - Campaign Content : hyperlink / calltoaction / confirm

For Shipment tracking email

 - Campaign Source : website 
 - Campaign Medium : email 
 - Campaign Name : order_shipped 
 - Campaign Content : hyperlink / calltoaction / confirm


Please Note: *And the end of the day all fields naming does not matter because you'll be know for want campaign you are running those email, etc, etc.*

Hope this helps!