Modbus16 is a CRC16. CRC calculations have several parameters:
- the bit width, in this case 16
- the polynomial, in this case 0xA001
- the initial value,in this case 0xFFFF
- the bit order
- whether the final CRC is inverted with an XOR.
There are quite a number of CRC16s defined, with different values for these parameters, and this appears to be one of them. See the Wikipedia article on Cyclic Redundancy Checks for more informaton.