The mSATA SSD can be used as a separate harddrive, or as an expresscache drive, which is used to boost the performance of the main hdd by storing some frequently used files in the mSATA drive. From one of our forum user opinion, the speed is never going to be as fast running the OS directly from the mSATA SSD or from the SSD installed into the system.
However, Lenovo does not ship the mSATA SSD (if ordered with the ThinkCentre M92z) with the OS and recovery partition installed. If you want to use the mSATA as the main harddrive, you have to install the OS on the mSATA yourself.
The reason why OS is not installed on the mSATA SSD is due to space restriction. With the recovery partition and the OS + drivers installed it is already 40GB gone from the 64GB mSATA, which makes it useless as a main drive.