“…a reaction (chosen from a baseline to increasingly rash/cautious, depending on the colonel) is compulsory – despite the fact that the approaching troops are friendly reinforcements…” Does this mean that a unit that fails command control roll has to react to anything in its zone of control?
Any unit which is not under command (by whatever sub-system the rules mandate – out of range; no command pips allocated; fails a die roll etc.) must roll to check if it reacts to any troops which approach within a rules specified distance of it (a zone of control, if you like). The unit will either pass or fail that roll. If it fails it will do nothing for the remainder of the turn – even if shot at by the unit which caused it to test its reaction.
If it passes, it rolls a second die and adds the colonel’s rating. The resultant modified die roll is the least aggressive thing a bold colonel’s regiment will do (so a modified roll of 3 compels the regiment to stand and fire, advance and fire, advance or charge – player’s choice). The roll represents the most aggressive thing a cautious colonel’s unit will do – because the numbers run the other way. A 3 would compel a cautious colonel’s unit to advance and fire, stand and fire, withdraw or retreat (see tables in my original message).
There are other bits (poorer colonels require greater effort on the CiC’s behalf to put them under command; artillery batteries are always cautious; once shot at, a unit not under command is more likely to react; units not under command can move but not enter other units’ zones of control; units react to the closest of multiple units in their zones etc.) but that’s a broad outline of how it works.