Figure 1-5 shows how the µC/Modbus communications stack fits in your product and also shows which source files are associated with each layer.
MB stands for ModBus, MBS stands for ModBus Slave and MBM stands for ModBus Master. A file that starts with mb_ indicates that the code in the file is independent of Modbus Slave or Master. A file that starts with mbs_ contains Modbus Slave specific code and, of course, a file that will start with mbm_ will contain MODBUS Master specific code.