In this scenario the artefact in the core folder has been replaced with the upgraded artefact and the customer does not want to use this artefact. The customer is currently using the core artefact i.e. no changes have been made to the artefact by the customer. The customer must keep the pre-upgraded version in the custom folder as this is now different from the core solution. The build script will always take the artefact in the custom folder before the core folder.
An example of this is a change to the DW_ADDRESS_ETL meta data. There has been an upgrade to this ETL to include a new destination field. Since the customer wants to use the original pre-upgraded version without the new destination field they must keep the pre-upgraded version of the DW_ADDRESS_ETL meta data in the custom\core\etl folder. They can do this by exporting the ETL from the ETL tool to this folder.