Conformity specifies the amount of resistance a thermometer is allowed to deviate from a standard curve (such as the curve produced by the Callendar-Van Dusen equation).
A tolerance at the reference temperature is usually 0ºC, and a tolerance on the slope or TCR.
Below shown figure states that a resistance thermometer conforms most closely to its curve at the reference temperature, while the resistance fans out above and below this reference.
Interchangeability between two thermometers is no more than twice the value of their Conformity. Commercial platinum resistance thermometer elements are available with extremely tight tolerances, to within 0.026ºC in some cases.
When interchangeability is an overriding consideration, the specified may consider other means to achieve it. For example, manufacturers may alter their calibration procedures to fix the reference temperature and tightest tolerance at a point other than 0ºC.