Bane Weapon Fusion
(from Starfinder Core Rulebook)
The bane fusion enhances the power of critical hits against certain foes. Against a designated foe, the bane weapon gains the stunned critical hit effect. If the weapon already has a critical hit effect, when you score a critical hit against an appropriate foe, you can apply either the weapon’s normal critical hit effect or the stunned effect. When you add this fusion to a weapon (or create it as a fusion seal), you must select a single creature type (aberration, animal, construct, dragon, fey, humanoid, magical beast, ooze, outsider, plant, undead, or vermin) against which it will apply. If you select humanoid or outsider, you must also select one subtype. The stunned critical hit effect applies only against creatures of the selected type (and subtype, if selected). Once selected, the creature type and subtype can’t be changed.