I suspect the Keystone is a Cipher to decrypt the data somewhere or to run a process which is publicly available.

Taken from wikipedia - Secure CryptoProcessor -

"The purpose of a secure cryptoprocessor is to act as the keystone of a security subsystem, eliminating the need to protect the rest of the subsystem with physical security measures."

https://en.wikipedia.org/wiki/Secure_cryptoprocessor

Taken from another site

To further clarify how I understand the semantics: because the key is required for the algorithm to function at all, the key is 'meant' for the algorithm (e.g. a keystone for an arch or a punch card for a IBM computer) the term 'key' is meant as an enabler for a process, not as a lock/unlocker for a secret. This is especially true because cryptographic keys can be used for algorithms that do not 'hide' or 'protect' data.

https://crypto.stackexchange.com/questions/25364/definition-of-the-term-key