The secret key implemented by NetSuite allows Connector for NetSuite to encrypt and decrypt information it sends between AvaTax and NetSuite, keeping your data more secure.
You secret key can be any combination of 16, 24, or 32 letters and numbers. You won't need to remember it or enter it anywhere else, so you don't need to make it memorable or write it down. If you uninstall and re-install Connector for NetSuite, you just enter a new secret key.
And you don't need to worry about whether or not someone else in your organization has already entered a secret key. The page prompting you to enter a key only displays if a key has not yet been entered. Once that's done, you won't see the page again.