When you hit the captain icon on a booked worker, you will now be able to choose the type of captain you'd like to assign them as:
Shift Captain - In charge of all workers under a specific shift.
Area Captain - All shifts and workers under the same area in an event (set on the Edit Shifts screen). You can also assign the same Captain to multiple areas!
Event Captain - All workers under a specific event, regardless of shifts and areas.
If you have a worker set as a Default Captain in their profile, they will automatically be assigned as an Event Captain once they are booked.
All the information you need to know about captains is here.