Thermocouple Grade Vs. Extension Grade

What is the primary difference between Thermocouple grade wire and Extension grade wire here? http://www.omega.com/pptst/TT-J-TWSH.html

There are both for making T/C longer? I don't understand the difference.

The unstated difference is that extension wire doesn't quite meet the accuracy spec. It's good enough to provide a contiguous gradient at lower temperatures, but not good enough to qualify for standard limit of error wire.

As the Omega commentary states, you can use T/C wire for extension purposes, but you don't use extension wire for the T/C sensor itself.