Class 11 material is a designation of the Resistance Welding Manufacturers Alliance (RWMA). It describes a Tungsten/Copper base material with good conductivity and excellent strength at high temperatures and forces. Class 11 is often used as a back up electrode for projection welding.
Two typical applications for Class 11 are automotive nut welding and spuds on water heaters. These applications generate large amounts of heat and mechanical wear on the electrode face. The combination of copper and tungsten handles this very well for a long time with little maintenance.
The product description and properties of Class 11 can be found in the Resistance Welding Manual Section 18-3, available at RWMA/AWS.
There are other Class materials with other properties shown inthe above references. Their properties allow for welding work pieces with applications other than the high heat and forces that class 11 normally encounters.