Class 20 material is a designation of the Resistance Welding Manufacturers Alliance (RWMA). It describes a copper base material with high conductivity and good strength, similar to Class 2. In addition it has better resistance to annealing and may have less tendency to stick to the work piece. Class 20 is most often used to weld galvanized materials.
The product description and properties of Class 20 may be found in the Resistance Welding Manual section 18-3, available through RWMA/AWS.
There are other Class materials with other properties all shown in the above reference. Their properties allow for welding work pieces with properties both higher and lower than galvanized steel.
Reference: RWMA - Resistance Welding Manual 4th Edition
AWS J1.3 - Specification for Materials Used in Resistance Welding Electrodes and Tooling