Doubts in the file PacketRegister.cpp

[UT00801]

Hello everybody: I'm trying to add new features to DCCpp. In the PacketRegister.cpp file, I do not understand the use of the "callBack" and "callBackSub" variables that are read from the serial command received in the "readCV", "writeCVByte" and "writeCVBit" functions. Someone, could tell me what its function if it has it?

Thank you very much in advance.