Playing with mapping data from one table to another and finding it quite confusing!!! I understand now after consulting help pages, but I think this little note (first screenshot) from the click tour captures the confusion pretty well: "Enter a placeholder character because we're not doing the thing the action description says!"
I suggest changing the action description (2nd screenshot) to "Write a list, object, or set of values/columns to another Clay table as a new row." and also display the "Add other data from this table" section of the modal window before the user selects the table to write to.