Each time an ASTRA MILITARUM unit from your army (see below) is selected to shoot, you can select one enemy unit within 18" of and visible to that unit. If you do, until the end of the phase, models in that ASTRA MILITARUM unit can only target that enemy unit (and only if it is an eligible target) and that enemy unit is caught in overlapping fire. While an enemy unit is caught in overlapping fire, each time a GENESTEALER CULTS model from your army targets that enemy unit with a ranged attack, add 1 to the Hit roll.
You can include ASTRA MILITARUM units in your army, even though they do not have the GENESTEALER CULTS Faction keyword. The combined points cost of such units you can include in your army is:
A GENESTEALER CULTS model must be your WARLORD, and ASTRA MILITARUM models from your army lose the Voice of Command ability if they have it. You cannot include units with any of the following keywords in your army using this rule: AIRCRAFT; COMMISSAR; EPIC HERO; MILITARUM TEMPESTUS; OGRYN; RATLING; TECH-PRIEST ENGINSEER; MINISTORUM PRIEST.