Ilhām means an inward receiving of meaning. Correct understanding may sometimes occur in that manner, but such an experience cannot be made into a universally binding, independent and infallible means of knowledge. Maintaining this boundary requires distinguishing prophetic revelation, solitary testimony, juristic following, probability, and decisive knowledge.
وَإِلَّا فَلَا وَجْهَ لِحَصْرِ الْأَسْبَابِ فِي الثَّلَاثَةِ
Otherwise there would be no basis for restricting the means to three.
The threefold restriction depends on the intended technical senses of knowledge and an authoritative means. Those distinctions clarify the limits of the threefold classification and the different roles of personal inspiration and generally authoritative evidence.