Key exposures are a feature that can help preserve the exposure of specific drawings at specific frames of your scene when you perform drawing substitution.
Before working with key exposures, it is important to understand how exposure works with Harmony. An exposure basically represents the span of time (or frames) during which a specific drawing is displayed in your scene. You can see the exposure of each drawing for a given layer in the Xsheet view. For example, here we have a layer in which drawing 1 is exposed from frames 1 to 6, then drawing 2 is exposed from frames 7 to 10, and drawing 3 is exposed from frames 11 to 12:
As you can see, in the Xsheet view, an exposure is represented in its first frame by the name of the drawing, then in every subsequent frame by a vertical line. The number represents the moment where the exposure of a drawing starts, which is called a key exposure, the line represents the span during which this drawing's exposure is repeated, which is just called exposure.
Typically, key exposures are automatically created when the exposed drawing changes. However, it is possible to add key exposure in the middle of a drawing's exposure without changing the exposed drawing, simply to break its exposure into two separate blocks of exposure. For example, here, you have drawing 1 exposed from frames 1 to 8, but it is split into two blocks of exposure by a Key Exposure at frame 5.
This is useful for managing what happens when you perform drawing substitution. In Harmony, when you swap a drawing for another drawing at a specific frame, it replaces which drawing is exposed on all the frames starting from the current frame up until the next key exposure. For example, here, if we switch the drawing at frame 3 with drawing 6, drawing 6 will be exposed from frame 3 all the way to frame 8, because there is a key exposure at frame 9.
In this other example, because there is a key exposure at frame 5, if the drawing at frame 3 is switched from drawing 1 to drawing 6, the substitution will only happen on frames 3 and 4. The key exposure at frame 5 will prevent the substitution from spanning any further:
Hence, by using Key Exposures, you can make sure the drawing exposed at a specific frame does not get swapped just because you swapped the drawing in earlier frames. The drawing exposed with a Key Exposure only gets swapped if you select the frame where that Key Exposure is and swap the drawing in it.