In a word, the PIN. The "Product-ID" and "Encryption-Key" Options used by the KeyBuild program to determine the Product PIN for your protected Product. And your program must specify this exact Product PIN when performing a KeyChk function to check for the presence of your Key.
In order to build a "replacement" Key for your Product, another EverKey User would have to know the Product-ID and Encryption-Key that you used when building your Keys. Although the Product-ID is easily determined by using the KeyShow program, the Encryption-Key is only stored in your Options file. This is why both the Product PIN and Encryption-Key should be treated as a company SECRET.