When decoding a 32-bit IEEE 754 float carried over 2 consecutive Modbus registers, how many different byte-order combinations does a developer typically encounter in practice depending on the slave?

Four combinations are commonly seen: ABCD (standard Big Endian, the official Modbus order), DCBA (reversed Little Endian), BADC (Big Endian Byte Swap -- registers in the right order but inner bytes swapped) and CDAB (Little Endian Byte Swap -- registers swapped but inner bytes in normal order). A is wrong because, in practice, far more than one order is encountered. B is wrong because there are not just two orders; both byte-swap variants must be supported as well. D is wrong because, although eight permutations of four bytes exist mathematically, only those four are actually used by vendors. The root cause is that the original Modbus standard only specified byte order within a 16-bit register, not how to combine two registers for a 32-bit value, so each vendor made its own choice.

Configure your driver with the byte order as a parameter, not as a constant in the code: changing vendor or firmware version will eventually force you to switch convention without recompiling.