A scale is convex if every octave equivalence class contained in the convex hull of the classes of the scale is itself a class of the scale.
In other words, Gene is proposing a strict limitation on how far he'll allow you to transpose notes of a fokker periodicity block by its unison vectors [as described in Paul Erlich, A Gentle Introduction to Fokker Periodicity Blocks -- an excursion] before the periodicity block ceases to look 'blocky' anymore.
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