Thermocouple grade cable is used internally in sensors to create a sensing point, extension cable is used to connect a sensor to the measuring instrument. It is actually the same as Thermocouple grade cable which means the chance of error is significantly reduced.
Generally recommended using no more than 80 - 100 metres as this will start to cause errors in the reading, it is also recommended that shielded cable is used for longer runs of cable. For more detailed technical information on extension cable