Source: Starfinder Core Rulebook p.104
As a move action, you can cause all of your melee attacks to deal fire damage instead of their normal damage type. (The attacks are still made against the target's EAC or KAC as normal for the weapon.) This benefit lasts for 1 round or until you leave photon mode.
When you are attuned or fully attuned, your attacks with plasma sheath deal additional fire damage equal to half your level.