Not every factory uses the word viso the same way, but in common trade talk it is the band of leaf above seco and below the true ligero. It carries more oil and more body than the middle of the plant without being as stubborn as the crown.
Some houses skip the name and sort only three filler grades. Others keep viso as its own pile so a blender can add weight without piling on ligero. Either system is a sorting choice, not a law of botany.
When a tasting note talks about “more viso in the blend,” read it as a construction claim: thicker mid-to-upper leaf, slower burn, more body. It is not a flavor science.