Organizing omission by the missing element and communicative purpose reveals why equally short expressions may achieve quite different rhetorical effects.
Motives for omissionPrimary function
1–2: secrecy, deniabilityConceal identity or permit denial
3–4: unique reference, attentivenessPoint to a clear referent or test attention
5: constrained occasionRespond to distress or urgency
6: honour or contemptConvey esteem or aversion
7: metre and cadencePreserve rhythmic structure
8–9: generality, tactBroaden scope or avoid indelicacy
10–11: no specified object, passiveEmphasize a verbal quality or event
General analytical rule: First reconstruct the intended meaning, then identify the contextual cue, and finally explain the specific motive. Thus “ghazāl” concerns urgency, “wa-mā qalā” concerns cadence, and “yaʿlamūn” highlights the quality of being knowledgeable.