Swarm Yellow has 20 less health and one extra charge than the normal Yellow Bloon. Usually, decks with Swarm Yellows will also have normal Yellows, for more early-game bloons. Most of the time, it's used for aggro, and also has good synergy with Zany Juice.
Some players may equip Swarm Yellows purely for extra Bloontonium per card while still having the opportunity for early-game pressure.
Unlike normal Yellows, a Swarm Yellow can be one-shot by Storm Of Arrows.