What this catalog line is
The Mitsubishi FX3S-20MT/DS is a base-unit PLC in the FX3S family, configured with 12 inputs and 8 transistor sinking outputs. It is filed in this catalog as a PLC base unit under the FX3S series — not an I/O slice, expansion module, or HMI panel.
Where this class of unit goes in a panel
An FX3S base unit like the 20MT/DS typically anchors a small, standalone machine control section — the CPU, the on-board I/O, and the programming port live in one package, so it suits OEM cells where a compact PLC and a handful of digital points are all that is needed. Because the outputs are transistor sinking, the 20MT/DS pairs naturally with DC loads wired to a common positive rail rather than AC coils; specifying it into an existing panel that was built around relay outputs usually means a wiring rework at the output terminals, not just a CPU swap.