There's some history here.
In traditional modbus addressing, the format is five digits, where the first digit is a prefix for the memory area: '0' for writable coils, '1' for readonly discrete inputs, '3' for readonly input registers, and '4' for writable holding registers. { There's also '6' for extended file registers, but that is not supported by most devices and drivers. }
The key point is that the leading '0', '1', '3', and '4' are prefixes, not otherwise treated as numbers.
So the patterns were 0xxxx, 1xxxx, 3xxxx, and 4xxxx. The "x" is a decimal integer, originally constrained to 1-9999 (four "x"s). The underlying wire protocol uses a 16-bit binary number to carry "x", and it starts with zero instead of one.
The discrepancy between displayed "x" and "x" on the wire, with adoption by multiple brands before the protocol was really standardized, created some varying implementations.
First, some implementations wanted a nice even ten thousand registers, but still in five total digits, so they made their stuff start with "x" = 0, both on the wire and as displayed.
Then, other implementations wanted to use the full 16-bit range (including Modicon themselves), so they made it six total digits, with "x" from 1 to 65,536.
Then, some more implementations adopted the six digit format, but from 0-65,535, identical to the number on the wire.
Ignition replaces the confusing numeric prefixes with alphabetic prefixes, and therefore allows you to omit the no-longer-needed leading zeros in "x". Ignition still defaults to "x" starting with one, but there's a driver setting to use "x" starting from zero.
Ignition also allows variations on the alphabetic prefixes to manipulate the values on your behalf, including interpreting multiple registers together.
