Unique elements

ISO20022 describes StatusIdentification, OriginalInstructionIdentification, OriginalEndToEndIdentification and OriginalTransactionIdentification (StsId, OrgnlInstrId, OrgnlEndToEndId, OrgnlTxIdas) as follows:

Presence: [0..1]
Definition: Unique identification, as assigned by an instructing party for an instructed party, to unambiguously identify 
the reported status.
Usage: The instructing party is the party sending the status message and not the party that sent the original 
instruction that is being reported on.
Data Type:  Max35Text, Format:  maxLength: 35, minLength: 1
To conclude, the element is optional by schema and it has to be unique. Uniqueness is checked by the validator within one message.