UL94 is a plastic flammability standard that rates the ability of a material to extinguish or spread flames once it has been ignited.
Thermal conductivity measures a material's ability to conduct heat.
Thermal impedance is the reciprocal of thermal conductivity. Thermal impedance and resistivity mean the same thing.
Factors such as material type, thickness, or surface irregularities (causing microscopic air gaps) impacts thermal impedance.
Thermal impedance of Thermal Interface Materials is often measured according to the ASTM D5470 standard.