There's really not that much of functionality:
- Three digits display with movable decimal point.
- Three status leds.
- Quadrature encoder with button-functionality.
- Communicate with host via i2c/serial-ttl.
- Multiple units can be chained.
- The value can be output as an analogue voltage with 12 bit resolution.
It looks like a lot of parts but that probably just an illusion :-)
Starting from top there's 4 n-fet transistors for muxing the cathodes of the displays and leds. Then there are 8 anode resisors.
The atmega with C3/C4/C5 decoupling caps.
The crystal with C1/C2 load caps is followed by R10/C7/C6 that is a filter for the supply to the mcp4821 12 bit dac.
Below the atmega there are some jumper for selecting if the units is to communicate with serial uart or i2c.
The two 4 pin headers can be used to chain multiple units either by i2c as a bus, or a daisy-chain with serial, if serial the last unit in the chain must be looped.
R9/R11/R12 is pullups for i2c and reset.
The 2-pin header is analogue voltage out and is filtered by C8.
The 5 pads on the bottom is the programming header.