PCO stands for Prototype Concept. It holds the state machine information for a class of concept. So, every concept (CO) has its state machine set by the PCO when it is created. If you tell a CO to change its structures, it tells the PCO which does it for the CO.
NOTE: structures can not be changed after initialization, though I don't know if I check this in all cases in the code.
A PCO is created from the chasmxml file.
A PCO tracks all the COs of itself. It is also necessary for the ChasmGui.