What is the main difference between class and group 3 fax modem?

Answer

The most common fax classes for fax modems are Class 1 and Class 2 where Class 1.0 and Class 2.0 are standardized by the ITU-T.These classes are an extensions to the basic Hayes command set. Class 1, 1.0, 2 or 2.0 describe the interface of the modem to the PC. Group 3 fax however is the communication standard between fax machines.