What is a key structural condition of the inserted material in iʿtirāḍ?

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

• It must be a direct object
• It must have an accusative clause-position
• It comprises one or more clauses without a host-syntactic position
• It must always contain a vocative