Instruction keys are used when you have to tell your bank to indicate specific parameters (parameters are define by banks).
For example, in Belarus, they have parameters whether payment is urgent or not.
So we define two instruction keys for field 1.
Instruction key 1 has code word 'X' - meaning that paymet will be urgent.
Instruction key 2 has blank code word - ' ' - meaning that payment will not be urgent.
We have "Urgency" field in our DMEE tree structure.
So the next time when user define urgency - the code word will be transfered from FI document through DMEE tree to payment file. And bank will know what urgency will have our payment.
Also we use other 3 instruction key fields - for them in DMEE tree structure we also have 3 payment parameters fields.