The FX3U-16MT/DS is the base main unit in the Mitsubishi MELSEC FX3U family — 8 digital inputs and 8 transistor (sink) outputs on a 24 VDC supply, 25 W draw. It scales to 128 I/O with expansion modules, so you can match the card count to the line instead of swapping the whole CPU.
Transistor sink outputs — what that buys you
8 transistor (sink) outputs switch the load to ground. No mechanical contacts to wear.
The FX5-8EX/ES is an input-only expansion module for the MELSEC iQ-F series, not a main unit. It adds 8 DC inputs (sink/source) but needs an FX5 CPU to talk to. The FX3U-16MT/DS is a self-contained controller — you can't swap one for the other without changing the backplane and the programming environment. If your BOM says FX3U-16MT/DS, the FX5-8EX/ES isn't a second source; it's a different animal.