Modicon 984 ladder logic is basically a free structure program.
1x bits (sometimes inacurratley called registers) are read only and are used for real world inputs.
0x bits (also sometimes inacurratley called registers) are read write and are used for external outputs or as internal memory coils.
3x words are read only and are used for real world inputs such as analog inputs or BCD numbers.
4x words are read write and are used for external analog outputs, memory storage, timer/counter values, and other operations requiring words instead of bits.
Addresses are assigned to real world points in the "traffic cop". Addressing is not dependent on card type or location, does not need to be consecutive, and can have "gaps" (i.e. output card 1 = Ox97 to Ox112, card 2 = 0x1 to 0x16 allowing 0x17 to 0x96 to be used as internal coils). Numbering is decimal and begins at 1. On new systems it is the software setup that dictates if the address is 6 digits (4xxxxx) or 5 digits (4xxxx). Timers and counters do not have dedicated addresses so 4x words/registers are required for preset, current, and elapsed values.