Dominions 5 Wiki

A Corpse Stitcher is essentially a Healer for the Undead. Their job is to repair structural damage that persists after battle, also known as Battle Wounds or Afflictions. The point value shows how much work they can do each turn.

Note the name of the trait, "Corpse Stitcher". A Spiritform does not have a corporeal "corpse", and thus cannot be mended by a Corpse Stitcher. They can't reattach a Ghost's arm, for example.