If I remember right Flash is the final command learned from that materia. It starts with Double Cut, then 4x Cut, and finally Flash, which allows you to attack all enemies at once(one hit to each enemy). I always found 4x Cut more useful as there never seem to be enough situations where you fight 4 or more enemies
Correction. Flash is the 2nd lever of Slash-All Materia. 2X Cut goes to 4X cut on it's next level. That is all, they are separate materia.