What this line is
The FX3S-20MT/DSS: For a panel-builder or controls integrator, the FX3S base unit sits at the bottom of the FX3S stack — small-format machine control, typically mounted on DIN rail with the I/O wired directly to its fixed terminal block. The MPN follows the Mitsubishi convention used across the family: the trailing 'MT' denotes transistor outputs and 'DSS' the DC supply / sink-source output class, which sets the output wiring topology a controls engineer has to land on the schematic.