Think of it as specifying the cause and effect of this transaction. Or, depending on the interpretation, it may be the primary target of this transaction. It's about choosing which item the money came out of and into which item, or which item was increased and for what reason. Items can be selected for only those of the account set in advance in the environment settings.
In accounting, it is called debit/credit. In fact, this selection is a bit difficult, but in whooing, there is no big difficulty because it leads to situations that can be used a lot in individual situations.
Please refer to the below for detailed selection of left/right items by transaction type.