In this scenario the artefact in the core folder has been replaced with the upgraded artefact but the customer does not want to take on these upgrade changes. The customer has already made changes to the artefact which is in the custom folder. No modifications need to be made as the build script will use the custom artefact and ignore the upgraded artefact in 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. However the customer has already modified the meta data by deleting an existing column which they do not need. Because the customer does not want to adopt the upgrade no modifications need to be made and the build script will still use the ETL in the custom folder and ignore the upgraded ETL in the core folder.