Brief question on roman history?

This probably relates to the organisational structure of his forces during this time. He probably organised the legions to perform certain roles at certain times. The cohorts probably refer to the re structuring of some legions into smaller bodies of men. Smaller bodies of men can move into areas on the battlefield that would be impossible for a large formation and would ist in the breaking of enemy lines, flanking manouvres and in planning where to place support troops. As we know cohorts formed a legion it would be logical to accept this description as a diffusion of a legion into component structures that perform the same function.