The RelatedInstrumentGrp is a repeating component at the same hierarchical level as the Instrument component, describing relationships and linkages between the Instrument, UnderlyingInstrument and InstrumentLeg entries. If all instances of the UnderlyingInstrument in the message are true underliers of the Instrument then the RelatedInstrumentGrp component is not needed. If any instance of the UnderlyingInstrument has a different relationship, e.g. underlier of an InstrumentLeg, stream, equity equivalent or nearest exchange-traded contract or there are multiple instances of InstrumentLeg, then an entry for every relationship should be included in this component. When the RelatedInstrumentGrp appears within a repeating group, each entry only apply to the Instrument component at the same hierarchical level.