Makeup Water Tank

It's where people go who have been fighting?

Or, it holds water to replace the water that evaporates from the chiller?

I'll go with #1. It's the designated timeout spot for people that have been fighting, to make up.

For the correct answer see the Control Philosophy document for the plant, which should contain sufficient information to explain both its purpose and the basis for its sizing.

if it says "make up" it usually implies water is being used in a process causes water loss, the make up replenishes that lost or used water. it could also be thought of "supply water"

It could also be thought of as an expansion tank. After all, the pipework is changing volume with temperature, as is the fluid, and accomodating the volume fluctuations would be useful. The forum still doesn't know what is in the Control Philosophy document for this plant. Please explain.

Does this chiller plant have glycol in the loop?

If yes then they sometimes install make up tanks (volumes vary on size of loop) with the proper glycol solution, so when the pressure drops from air loss the tank has a pump interlocked to a pressure switch which will make up for lost pressure.