The aquaero5 board has several inputs and outputs, but not all of them need to be connected. For example, the aquaero5 version I have, has 8 physical connectors for temperature sensors, the board comes with only 4 sensors (you can buy more if you want) but the firmware reports even more temperature sensors present (16, only 8 are physically present in the board). The AQ5_FLOAT_UNDEF (value -99.0) is used by the firmware to report that the value is not present. I think that the right thing to do is to not report sensors that are not connected.
The same applies to fan controllers. One thing is to have a fan blocked in 0 rpm, and another thing is to disconnect that fan.
 
#define AQ5_FLOAT_UNDEF			-99.0