This subsystem implements only the POSIX.1 standard – also known as IEEE Std 1003.1-1990 or ISO/IEC 9945-1:1990 – primarily covering the kernel and C library programming interfaces which allowed a program written for other POSIX.1-compliant operating systems to be compiled and run under Windows NT.
That is, Windows NT did not provide a POSIX shell nor any Unix commands out of the box, except for pax.
The NT POSIX subsystem was included with the first versions of Windows NT because of 1980s US federal government requirements listed in Federal Information Processing Standard (FIPS) 151-2.
[1] This standard required that certain types of government purchases be POSIX-compliant, so that if Windows NT had not included this subsystem, computing systems based on it would not have been eligible for some government contracts.
Windows NT versions 3.5, 3.51 and 4.0 were certified as compliant with FIPS 151-2.