It can refer to Ripperdactyls being placed into the skies from the Shadowstrike Starhost battalion (Seraphon Battletome) or to units placed into the Havens from Dracothion's Tail Starhost battalion (GHB2019).
Some units can be kept in reserve. You pay the points cost of those units BUT you don't set up them on the battlefield, you simply put them apart; later, when you decide their moment has come, you field them following their specific rule (usually explained in the battalion warscroll or in the general rules of the army). "placed in heavens" is the Seraphon's term for units placed in reserve. Other armies can do the same (Stormcast Eternals, Nighthaunts, Legion of Nagash and so on)... the mechanic is more or less the same, but the names to identify the ability are different. EDIT: ...and welcome aboard!