If you are playing a matched play game, you are allowed to have an understrength unit in an Auxiliary Support Detachment. Due to constraints within Battlescribe, we are not able to reflect this in the error checking. To properly calculate your PL and Points, add the unit you want to take as an understrength unit and then open the context menu on each model in the unit you do not want to use via right click or tap and hold. Then delete the model from that context menu. This will permanently display an error in your list, but it will calculate your points correctly.
If your army is Battle-forged, all Troops units in ASTRA MILITARUM Detachments and all LEMAN RUSS units in Spearhead Detachments gain this ability. Such a unit that is within range of an objective marker (as specified in the mission) controls the objective marker even if there are more enemy models within range of it. If an enemy unit within range of the same objective marker has a similar ability, then the objective marker is controlled by the player who has the most models in range as normal.