From Trigger Magazine 04:

Using the cut above as another example, the divided cell each represent an individual motion with in the scene. The shockwave/explosion effect is represented by the "A" cell, Satsuki by the "B" cell, and Ryuko by the "C" cell. To those who are familiar with photoshop, they might find it similar to the layer divisions.

こちらのカットの様に、分けられたセルは全て別々の動きをします。衝撃エフェクトのAセル、皐月のBセル、流子のCセルと若いアルファベットを奥に順番に配置されていきます。簡単に説明するとフォトショップのレイヤーに近いかもしれませんね！