A custom key is the column or combination of columns which attempts to distinctively identify every single row, if there is more than one row with the same key, those rows will be highlighted in gray. You can reconfigure the custom key without extracting the data again.
There are three options to create a custom key:
1) Extract the data, then press the custom key button to select the column(s) desired.
2) Run the “Extract and compare” step, when this happens and the “custom key” list is empty, JuxtAPPose will suggest by default to use the first column available and ask if that is correct, at this point the comparison can be canceled if that column is not intended to be used as the primary key of the data.
3) You can type the name of the column or columns (separated by commas) before starting any comparison, if any of the columns is not recognized among those from the data extract, an error message will be raised.