How does tatmīm differ from iḥtirās?

Ījāz, Iṭnāb and Musāwāh: Concision, Ellipsis, Expansion and Equivalence

Tatmīm adds a refinement; iḥtirās prevents an error
Tatmīm is just tanwīn
Both always involve deletion
Iḥtirās is necessarily useless