For IV administrations, I have seen that some EHRs represent their data as separate administrations of the main ingredients and diluents.  

For example, suppose a physician orders Potassium chloride 2 mEq/mL intravenous solution for their patient —which is represented as one administrations for Potassium Chloride, another for a Sodium Chloride intravenous solution, and another for Lidocaine. This means that in the MedicationAdministration resource, we have separate administrations for the Potassium Chloride, Sodium Chloride, and Lidocaine with the same start/end date for the same patient. We can link these administrations together using the MedicationAdministration.partOf field, however for downstream applications we want to be able to flag which of these medications is the main ingredient vs the diluent. We have this data in the EHR but I am not sure how to represent it in FHIR. Should this information go in the medicationAdministration.category field or in a separate extension?