In the PlantLoop object is an input for Load Distribution Scheme.Scheme. Choices are

PlantLoop,
  Optimal; SequentialLoad;          !- Load Distribution Scheme

Choices are:

A14, \field Load Distribution Scheme
    \type choice
    \key Optimal
    \key SequentialLoad
    \key UniformLoad
    \key UniformPLR
    \key SequentialUniformPLR
    \default SequentialLoad

This input is set up to allow a set of chillers (2 or more) to run the first few chillers at optimum part load ratio while the last chiller meets the remaining load. I have not tested this but it is supposed to work this way. I'm not sure what happens with only 1 chiller.