A

A Thermocouple grade wire is wire that is used to make the sensing point (or probe part) of the thermocouple. Extension grade wire is only used to extend a thermocouple signal from a probe back to the instrument reading the signal. The extension grade wire typically will have a lower ambient temperature limit in which the wire may be used. Namely, it may pass a signal representing a higher temperature as received from the probe, but the wire physically may not be exposed to higher temperatures. Thermocouple wire may be used as extension wire, but extension grade wire may not be used in the sensing point (or probe part) of  the thermocouple. Part numbers for extension wire typically begin with an "X" suffix.

A Standard vs SLE (special limits of error) wire has to do with accuracy of the wire. SLE wire is the same as standard wire with the added feature of having a little better accuracy spec. Accuracy of thermocouples vary with thermocouple types. e.g. For the lower temperature ranges type T, composed of copper wire in the positive lead and constantan (copper-nickel mixture) for the negative, has good accuracy specifications. Additional information on accuracy. 

A

A Thermocouple wire that is used in a thermocouple from the point of sensing to the point of cold junction compensation (cjc end) where the signal is measured. A thermocouple is a sensor for measuring temperature, that consists of two dissimilar metals that are joined together at the sensing end. Different thermocouple types (e.g. J, K, T, E, etc) use different mixtures of metals in the wire. At the cjc end, the millivolt value provided by the thermocouple represents the difference in temperature of the sensing end as compared to the cjc end (also called the reference end).