Priming means harvesting by height. Tobacco ripens from the bottom of the plant upward. Field workers pick the lowest leaves first (volado), then the middle leaves (seco and viso), and finally the top leaves (ligero). Each pass through the field is called a priming. The other method, stalk cutting, harvests the whole plant at once; it is more common for some filler crops than for delicate wrapper leaf.
The result is that one plant produces several different kinds of leaf. Wrapper, binder, and the various filler grades can all come from the same stalk if the leaves are good enough. Most of the leaf will not be good enough to use as wrapper.
Priming is a farming term. It explains why ligero leaf from the top of the plant is a different ingredient from the shaded leaves near the ground.