Laravel Custom Clause

Am trying to understand how the whereListing_type custom clause was created in Laravel but could not find any information on it so far

DB::table('property') ->join('auction', 'property.id', '=', 'auction.property_id') ->leftJoin('auctioneer_notes', 'property.id', '=', 'auctioneer_notes.property_id') ->whereListing_type('weauction')

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