What it will do in simple terms:do:

• Load each available equipment to the Optimum Part Load Ratio you have entered for each (if you have an availability manager that turns it off, it's not taken into account). See Chiller:Electric:EIR::OptLoad or Boiler:HotWater::OptLoad as examples
• Distribute the remaining load accross all equipment

This happens in PlantCondLoopOperation.cc#L2205