While a thermocouple wire type is often described by its chemical composition, the actual aim is to produce a pair of wires that follow a standardized  curve.
Impurities affect each batch of metal differently, producing variable Seebeck coefficients.
To match the standard behaviour, thermocouple wire manufacturers will deliberately mix in additional impurities to "dope" the alloy, compensating for uncontrolled variations in source material.
As a result, there are standard and specialized grades of thermocouple wire, depending on the level of precision demanded in the thermocouple behaviour.
Precision grades may only be available in matched pairs, where one wire is modified to compensate for deficiencies in the other wire.
A special case of thermocouple wire is known as "extension grade", designed to carry the thermoelectric circuit over a longer distance.
Extension wires follow the stated  curve but for various reasons they are not designed to be used in extreme environments and so they cannot be used at the sensing junction in some applications.
For example, an extension wire may be in a different form, such as highly flexible with stranded construction and plastic insulation, or be part of a multi-wire cable for carrying many thermocouple circuits.
With expensive noble metal thermocouples, the extension wires may even be made of a completely different, cheaper material that mimics the standard type over a reduced temperature range.