What is the essential difference between iḥtirās and tatmīm?

Iʿtirāḍ and the Relative Measure of Ījāz and Iṭnāb — Concluding Volume 1

• Iḥtirās is conditional and tatmīm always relative
• Both only concern poetic meter
• Iḥtirās blocks a misleading implication; tatmīm adds useful completion
• Iḥtirās forbids clauses