In restriction of a category, a general attribute or class is concentrated upon a chosen individual. In a sentence such as «Zayd is the emir», that may be the intended force. But if «the emir» refers to one previously identified officeholder, the real purpose may be to name that individual.
Examples:
«زَيْدٌ الْأَمِيرُ» — The type or the previously known emir; context decides.
«الْأَمِيرُ زَيْدٌ» — The known officeholder is identified as Zayd.
Both sentences involve a definite predicate or subject, but they need not serve the same informational goal. The particle al alone is not a complete interpretation.