It's revelant on servers, workstations, embedded systems, and supercomputers.<p>So, basically everywhere.<p>The thing is: you're unlikely to be programming directly on top of the POSIX API unless you're doing something comparatively low level to the average programming that is done in industry today.<p>Embedded is an area where applications are often written right on top of libc and POSIX interfaces. Even many RTOS which are not fully POSIX compliant borrow a lot of ideas from UNIX.