This refers to a transaction where an investment returns capital to the investor and doesn't have any accounting implications other than reducing the cost basis. The number of shares held is not changed.
A Return of Capital transaction can be entered in the stock register by entering the stock split with:
|Return of Capital value
The other side of the double entry would usually be a debit to the brokerage bank account.
It is not possible to use the Stock Split Assistant to do this type of transaction.
If you accidentally entered a non-zero price in the stock split,