Cray released several different OSs under the name UNICOS, including:
UNICOS: the original Cray Unix, based on System V. Used on the Cray-1, Cray-2, X-MP, Y-MP, C90, etc.
UNICOS MAX: a Mach-based microkernel used on the T3D's processing elements, together with UNICOS on the host Y-MP or C90 system.
UNICOS/mk: a serverized version of UNICOS using the Chorus microkernel to make a distributed operating system. Used on the T3E. This was the last Cray OS really based on UNICOS sources, as the following products were based on different sources and simply used the "UNICOS" name.
UNICOS/mp: not derived from UNICOS, but based on IRIX 6.5. Used on the X1.
UNICOS/lc: not derived from UNICOS, but based on SUSE Linux. Used on the XT3, XT4 and XT5. UNICOS/lc 1.x comprises a combination of
the compute elements run the Catamount microkernel (which itself is based on Cougaar)
the service elements run SUSE Linux
Cray Linux Environment (CLE): from release 2.1 onward, UNICOS/lc is now called Cray Linux Environment
the compute elements run Compute Node Linux (CNL) (which is a customized Linux kernel[1])
the service elements run SUSE Linux Enterprise Server