Position is the whole difference. A side tab sits on the vertical edge of the folder, so a full shelf presents a readable spine run to anyone standing in the aisle. That suits deep lateral cabinets and open shelving. Top tab labels do the same job for drawers viewed from above. The two are not interchangeable. See top tab labels, lateral filing labels and lateral cabinets.

Avery colour coding lateral filing labels displayed on office folders in a bright, organised desktop workspace.