Not every factory uses the word viso in the same way, but in common trade use it means the leaves above the seco and below the true ligero. These leaves have more oil and more body than leaves from the middle of the plant, but they burn more easily than the top leaves.
Some companies do not use the name and sort filler into only three grades. Others keep viso as a separate grade so a blender can add body without adding more ligero. Both systems are sorting choices, not botanical rules.
When a tasting note mentions “more viso in the blend,” it is a statement about construction: thicker leaf from the upper middle of the plant, a slower burn, and more body. It does not describe a specific flavor.