The marginal commentary explains that for earlier logicians, the logical definition of poetic discourse does not necessarily require formal meter or rhyme. Its decisive quality is the production of imaginative response.
Metrical verse follows prosodic measures. Imaginative discourse produces a vivid image or affect. These can coincide, but this logical classification emphasizes the second feature.
Therefore vivid prose may function imaginatively without meter; conversely, metrical lines need not be equally powerful as imaginative arguments.