What it sits as on the FX5 bus
The Mitsubishi FX5-C16EX/DS is an extension module in the MELSEC iQ-F family, built as a 16-point digital input slice wired from a 24 V DC source through a plug-in connector rather than a fixed terminal block.
Where it sits against the FX5 I/O family
The siblings in the same MELSEC iQ-F line serve a different job: the FX5-16EYR/ES is a 16-point relay output module, the FX5-16EYT/ES is a 16-point transistor (sink) output module, and the FX5-32ER/ES is a 100-240 V AC powered mixed I/O block with 16 inputs and 16 relay outputs occupying 32 I/O points on the PLC. None of those is a drop-in for an input-only extension, so the FX5-C16EX/DS remains the correct order code when the panel needs more 24 V DC inputs, not more outputs.