The Action field relates to the split rather than the whole
transaction. Consequently, if you fill in the Action field, the text will only show up in one account.

This is rather counter-intuitive from a user's point of view because the user will not be aware that a non-split transaction consists of 2 splits internally. After all, if you send a cheque from one account it will arrive as a cheque.

As it is a useful piece of information that could be used for filtering I suggest that the Action should show up in both accounts.

This could be achieved by automatically populating the Action field of the other split. They two Action fields would have to be kept in synch at every update.



Other information:
use case:
One of my current accounts pays me 1% cashback on all debit card
payments up to £10K. I enter "Debit Card" in the action
field. I created a report that selects all debit card pay-outs from
this account for a year. I use this to check whether I have reached my
limit and at the end of the year to check whether the bank has paid me
the correct amount.