What is the central structural difference between the prose and poetic cases?

Radd al-ʿAjz ʿalā al-Ṣadr: Repetition, Paronomasia and Derivation in Prose and Poetry

• Derivation is disallowed in prose
• Verse has no fixed closing position
• Prose uses opening/end; verse has four earlier positions
• Verse has only two lexical relations