Exactly
One target count only
Exactly 3 heads means only the outcomes with 3 heads count. Nothing with 2 or 4 heads belongs in the event.
At least
The target count and everything above it
At least 1 head means 1, 2, 3, and so on up to the maximum possible count. It is often fastest to use the complement, such as 1 minus the probability of 0 heads.
At most
The target count and everything below it
At most 2 aces means 0, 1, or 2 aces. This wording creates a range, not a single exact count.
Decision rule
Write out the allowed counts before you calculate
Before touching a formula, list the success counts that satisfy the event. If the list has one number, it is an exact event. If it has several numbers, you are summing a range or using a complement.