What most directly distinguishes tadhyīl from iʿtirāḍ?

Supplementary Practice 1: Applied Analysis of Faṣl, Waṣl, Ījāz and Iṭnāb

• Neither uses a clause
• Both only repeat a single word
• Tadhyīl reinforces a prior point; iʿtirāḍ is a parenthetical insertion
• Both necessarily have accusative syntactic position