As the title says, you will need to purchase the pass to continue progress on it when the Combined Arms operation launches next week. If you have already purchased the S5 pass, you will be able to switch between it and other passes as normal.
The Combined Arms operation will last five weeks (ends Dec 19, when Winter Contingency launches) and will have its own free 20 item track to progress on, a premium option will be available to unlock a premium-only item, keep the operation pass forever, award bonus xp during the duration of the operation, and to unlock an extra challenge slot for the duration of the operation.
For more information on Operations, see this Waypoint article.
But I used to be able to do the event AND still progress on the season pass at the same time. And that was with 4 challenge slots. Now, despite the fact that I bought the season 5 battle pass, I only get 3 challenge slots for the operations because I don’t want to fork over another $5 (each).
They could have just made events available for longer than 2 weeks but they chose to add more microtransactions. Idk how anyone with a brain could think that’s good for the players.