As a child I learnt how things work by pulling them apart, trying to put it back together again (if I was lucky) and figuring out how it all worked. Nowadays, I'm a bit more experienced and educated so I can quickly pick up how something works normally just by looking at it and recognising part number etc. I gather you have a circuit that is working but you're trying to understand how it actually works at an engineering level. I would suggest you read the datasheets carefully and break down the circuit into 'building blocks'.

From looking at the circuit, we have a DAC (current output I'm assuming) then we have U4a which looks like the current to voltage conveter (refer to the dac datasheet). At closer inspection, it seems that U14 is used to hold a constant current through the coil, this current is set by the voltage from the DAC circuit. The current is sensed across a resistor and the voltage across the resistor is fed through the pga into the a/d. At a guess, this may be used to measure the sensitivity of the unit under test. The polarity to the coil can be reversed via the relay - maybe to test the sensitivity in both directions? Then there's the mosfet connected to the 8254 - to provide a test frequency? Maybe related to speed testing the unit under test? I'm not an expert on testing relays, so I really can't comment.

Writing some simple code to feed a ramp into the dac and probing the various places with a CRO would confirm what is happening. I would suggest you brush up on ohms law, basic DC circuit theory and op-amps. If in doubt, take measurements with a CRO and/or multimeter and confirm your calculations. What would you do if there was no internet?