How does the system decide if a certain property is it exposed?
Does it have a flag in the property rule?
Does it check the System-Jobs-PropertyOptimization?
Does it check the corresponding table?
Is there any other mechanism to check?
PRPC will probably check if there is any instance of the Log-System-PropertyOptimization created for the property that you are using, This class 'Log-System-PropertyOptimization' is used to store the metadata of the properties optimized in the prpc system.
Pega Engine will maintain a class map which contains the mapping between the property name and the property class name i.e. Log-System-PropertyOptimization(pr_changelog). This table will store the entries of the optimized properties. So system will scan this table for the check of the property optimization status.