I am unaware of any FAR or DFARS requirement that the CLIN structure of the task order must match the structure of the basic contract. Some local contracting activities may have applicable local policy on the issue. Having the task order CLIN structure match the basic contract structure is ideal for tracking purposes but not always practical...depending on the complexity of the task order. While task orders are not truly stand-alone documents as they typically incorporate the terms and conditions of basic contract, they can contain unique aspects apart from (but not in conflict with) the basic contract. This includes line item structure.