When a unit has the reaction tag, can it be played in contested battlefields, or only in base/controlled battlefields?

Ruling: Playing a unit at reaction speed follows the exact same restrictions as playing a normal unit - only to your base or a battlefield you control.

Nuances:

• When attacking a battlefield, you don't control it, so you cannot play reaction units there
• When defending a battlefield, you do control it, so you can play reaction units there