Both functions use a declarative sentence. The difference is what new information the listener is supposed to gain.
PurposeWhat does the listener learn?
Fāʾidat al-khabarThe fact or judgment asserted by the sentence
Lāzim al-fāʾidahThat the speaker knows the asserted fact
Suppose a teacher already knows that a student passed. Someone saying “نَجَحَ طَالِبُكَ” might mainly convey that the speaker also knows. The same sentence to an uninformed person may convey the result itself.
The intended function is determined by the listener's prior knowledge and the situation, not by sentence form alone.