As explained in section 4.3 of the current Modbus specification, the Modbus protocol uses four different address ranges (called "tables" in the specification): discrete inputs, coils, input registers, and holding registers. The function code of a message determines to which address range the address in the message belongs. The address ranges may be separate, or they may be overlaid so that a register may appear in more than one address range. Note that the specification says nothing about how the overlaying is done - in particular, there is no mention of the byte-ordering issues involved.