why the "Num" field bleeds from one account to another

[Anna's unattended mail]

Why are the "num" numbers the same on all accounts that the
transaction touches?  That is, if I enter a transaction and the bank
numbers the transaction as "123456", I record that number in the
register for that bank.  However, outside of my control, this number
also appears on the expense account for the corresponding transaction.
I would like to use the order number on the expense accounts "num"
field.  Or I might want to create a unique number on the expense
report so auditors can correlate that to a receipt that would have the
same number.

What's the intent with "num"?