Selena's drive is Gunslinger. She has a cooldown gauge above her heat gauge that determines whether or not her ammunition will be infinite. If the gauge is full, then her ammunition gauge will deplete once attacks that involve her guns are used. Her attacks that involve ammunition during her drive are the only attacks that affect this gauge.
The cooldown gauge will decrease gradually whenever her drive is disabled.
During her Overdrive Gunslinger Redux, Selena's cooldown gauge will not increase, but rather decrease at a faster rate, and her ammunition gauge will temporarily remain infinite regardless of her cooldown or heat up level. The cooldown gauge will be empty again once it ends. She will also equip her visors once Overdrive is active.