Refer to the user manual of 11210, I would like to know why the worst case open circuit test set up for Null function is connecting the +ve terminal to DUT end while leaving the -ve terminal open ( Red terminal ) . Why not reverse ?

11210 uses AC signal to get voltage counts. If terminals are null, 11210 gets high voltage counts ; if +ve terminal is connect to DUT and leave -ve terminal open, 11210can eliminate the effect of parasitic capacitance. If -ve terminal is connect to DUT and leave +ve terminal open, there are still parasitic capacitors but contact check circuit won’t read the parasitic capacitors. So if users follow the suggestion in user manual,11210 can eliminate the effect of parasitic capacitance. Then 11210 will work well in contact check function.
Reverse connection will not happen this situation.