I managed to find some Wabco documents, and in it they explained briefly about ABS off-road (or TC in my case).

When in off-road mode, more wheelspin is permitted upto approx 15kmh more than the front wheels, to a maximum of 40kmh. Anything more and the TC engine control will take over. So it is not exactly TC-off, but TC-off-road mode.

Now... how do I explain to the inspector in layman terms?