Having 99 tracks per sequence gives you many options for “going vertical”—stacking clips in higher tracks to create effects that blend those layers of clips together. Compositing is the act of blending clips to create an effect. Compositing can involve a combination of opacity adjustments, filters (such as keys and mattes), or composite modes.
To keep things manageable, Final Cut Pro lets you create nests that collapse a multitrack effect into a single clip.
When compositing in Final Cut Pro, keep in mind that Final Cut Pro is a top-down compositor. The highest clip in the Timeline is processed as the foreground. Adjusting opacity, applying a filter, or changing a composite mode is usually applied to the higher clip to reveal ...