What is access protocol ACS?

Could you elaborate on what the Access Control System, or ACS, protocol entails in the context of cryptocurrency and finance? Specifically, I'm interested in understanding its purpose, key features, and how it is typically implemented in securing transactions and managing access rights in this domain. Is it a standardized protocol, or does it vary depending on the specific cryptocurrency or financial platform? Additionally, what are the main benefits and challenges associated with using ACS in this field?