What this base unit is, and what it isn't
The Mitsubishi FX3S-14MR/ES-K is a compact base-unit PLC from the FX3S series, configured with relay outputs for small-footprint industrial automation panels. It is filed as a PLC base unit, not an expansion module — the I/O count and CPU live on the single DIN-rail module, so when the BOM line says 'PLC', not 'PLC expansion', this is the form factor that ships.
Why the 14 I/O relay variant matters at the panel
The 'MR' designation and the 14 I/O figure drive the panel fit: relay outputs handle a mix of contactor coils and small loads without interposing relays, which is the usual reason a small machine builder picks this variant over the transistor-output siblings in the same series.