Definiteness, gender and number are all things an ism is. I'rab is different — it is what the ism is doing in this particular sentence, and it is the one property with no counterpart in English.
An ism stands in one of three states: رَفْع, نَصْب or جَرّ. Change the job and the ending changes with it. That is the whole of i'rab, and it is why Arabic can move its words around without losing the sense.
Four questions settle any ending — RICE. Role: what job is the word doing? I'rab: which of the three states does that job demand? Category: what kind of ism is it, and does it decline normally? Expression: how does that state actually show — a vowel, a letter, or nothing at all?
This page names the property and stops there. The RICE system in full — every role, every sign and every category — is section H of this guide.
Is i'rab about meaning or about grammar?
About role, which then decides meaning. The ending does not describe the thing named; it reports what that word is doing in the sentence. Change the ending and you change who did what to whom.
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