Tadhyīl characteristically follows a proposition with a clause that contains or reinforces its meaning. Iʿtirāḍ inserts a separate expression within the discourse for a distinct rhetorical purpose, without making it a constituent of the host syntax.
DevicePrincipal function
TadhyīlEncompasses or reinforces a prior proposition
IʿtirāḍProvides an independent aside within connected discourse
Some broader definitions may allow overlap, yet the ground of classification remains different.