Īghāl produces an added rhetorical effect near the close of a poetic line. Tadhyīl is not restricted to that poetic position. Its defining form is an additional clause that contains and reinforces the earlier meaning; īghāl need not take the form of a complete clause.
CriterionTadhyīlĪghāl
PositionAfter a preceding clauseAt the end of a poetic line
FormAn additional clauseNot necessarily a full clause
EffectReinforcement of earlier meaningA further effect at the ending
Tadhyīl is therefore not merely a special name for the last word of a verse.