When extracting data from Catalog, data sources can use the Select function.
Examples:
Catalog.Product – (simple form) extract from the Catalog Product all records, all fields
Catalog.Product | Select() – extract from the Catalog Product all records, all fields
Catalog.Product | Select ( id as product, name, title as product_title ) – extract from the Catalog Product only those fields that are necessary, set aliases, if necessary.