What defines a closed access control system?

A closed access control system is characterized primarily by its restriction on third-party integrations. This means that the system is designed to operate with specific components and software from a single manufacturer or within a proprietary framework, which prevents other systems or products from being integrated. This lack of flexibility can enhance security by minimizing vulnerabilities that may arise from third-party interactions, but it may also limit the system's adaptability to new technologies or functionalities that could arise from external sources.

The other options provided don't accurately define what a closed access control system entails. For instance, the ability for frequent updates and enhancements typically aligns more with open or hybrid systems, which are designed for adaptability and integration with various technologies. Similarly, strong integration capabilities are a hallmark of open systems, where compatibility with different vendor products is crucial. The lack of a user interface for management is not a defining characteristic of a closed system; even closed systems usually include user interfaces, though they may be limited in functionality. Therefore, the most correct defining feature of a closed access control system is its prohibition of third-party integrations.