Story Intersections is the name of a mechanic present in Resident Evil 6 that allows players from different campaigns to intersect with each other and start a small 4-player section.
Every campaign has at least one chapter where an intersection is possible, and when arriving at that point a one-minute timer starts during the loading screen. This is the given amount of matchmaking time the game has to find other players, the session host can skip this timer, which disables any guest from the second campaign to be able to join even if the session is set to public.
Story intersections are only available if the "Partner Join" option is set to "Allow".
The December 2012 title update added Agent as a co-op partner to Ada in her chapters, before this, the Ada intersections would only have 3 players max since her chapters were only playable in solo mode.
In Leon Chapter 5 and Ada Chapter 5, there is a checkpoint called "High-Rise Area" where Ada is inside a helicopter and Leon / Helena are on the ground, this section does not feature an intersection.
Likewise, in Chris Chapter 4 and Ada Chapter 4, there is a checkpoint called "Aircraft Carrier - Bridge" where Ada is sneaking around through the aircraft while Chris and Piers are chasing her down, this section also doesn't feature an intersection.