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.
An ASTRA MILITARUM Detachment is one that only includes models with the ASTRA MILITARUM keyword (excluding models with the AGENT OF THE IMPERIUM or UNALIGNED keywords). - If Every unit in an ASTRA MILITARUM Detachment has the MILITARUM TEMPESTUS, OFFICIO PREFECTUS and/or MILITARUM AUXILLA keywords, TEMPESTUS SCIONS units in that Detachment have the Troops Battlefield Role, instead of Elites - Troops units in ASTRA MILITARUM Detachments gain the Objective Secured ability (this ability is described in the Warhammer 40,000 Core Book).
If, in your movement phase, an OFFICER from your army disembarks from a TRANSPORT model, after that OFFICER's unit has been set up it can issue Orders as if it were your Command phase. However, when doing so, the Regimental Tactics ability does not apply, even if every model from your army has the ASTRA MILITARUM keyword. Note that an OFFICER model cannot issue Orders in this when disembarking from a destroyed TRANSPORT model.