you might be getting mixed up with a common pipework fault on your CCN1 course, ie a flued fire sitting in place but connected by a cooker hose (which would be classified as ID. AR or NCS?)As I understand the regs, only appliances that need to be moved i.e slot in cookers etc can have a flex, if a Hob is Fixed permanently into a work top ,then it should be connected with Rigid pipe