From Halopedia, the Halo wiki
Like normal Slayer, in Invasion Slayer the goal is to score more kills than the opposing team. Like Invasion, Invasion Slayer has a team of Spartans facing off against a team of Elites. A match is divided into three phases, however unlike Invasion the phases advance based on either team reaching a certain number of total kills or a certain time limit passing, whichever comes first.
As successive phases begin, new loadouts are available to each team, as well as better weapon and vehicle spawns. Rather than having static spawn locations like other game types, weapons and vehicles in Invasion Slayer spawn once a team has captured a control point, which will appear at various locations around the map over the course of a game. Once the point is captured by teammates standing in the designated area, a brief countdown begins, at the end of which a couple of weapons or a vehicle will spawn where the point was located.
Typically, then the control point moves to another location, however it may not reappear if there are too many vehicles on the map already. Once enough have been destroyed, another point will appear.