A serial number looks like 5234567890 or 2234567890, starts with a 5 or 2 and is entirely numeric. A product key looks like 9NGHJ-Z7TRQ-UTR12-F9TQP-KL7MN and is 25-character alpha-numeric.
New full product installation require a product key. However, for the setup of Upgrade editions a serial number of an older (previous) edition and a (new) product key will be necessary.
The serial number of an older (previous) edition can be seen in the 'splash screen' when starting the old product. Alternatively, once can start EndNote and choose 'About EndNote' from the 'Help" menu. The 25-character alpha-numeric product key will be communicated in an email message after an order is processed. Since a few months we have started to mention both items in the 'Order details' email message.