The following is the description of InstrId in ISO20022 Payments Maintenance 2009 document:

Presence:  [0..1]
Definition:  Unique identification, as assigned by an instructing party for an instructed party, to unambiguously identify
the instruction.
Usage: The instruction identification is a point to point reference that can be used between the instructing 
party and the instructed party to refer to the individual instruction. It can be included in several messages 
related to the instruction.
Data Type:  Max35Text, Format:  maxLength: 35, minLength: 1
By default InstrId check is on in the validator, so every InstrId in the xml-file has to be unique.