What is a subsystem in Windows NT 4.0?

In Windows NT 4.0, a subsystem is a component that provides a specific interface for applications to interact with the operating system's kernel. Each subsystem allows applications designed for different environments, such as Windows GUI or POSIX, to run on NT by translating their system calls into a format that the kernel can understand. Notably, the Windows NT 4.0 architecture includes multiple subsystems, enabling compatibility with various types of applications, thus enhancing its versatility and functionality.