What's the difference between a smart register booster fan and a smart vent?

A register booster fan (like the AC Infinity AIRTAP T4 or BIOWIND units) sits in or over an existing register and uses fans to push more heated or cooled air into a room that runs hot or cold. A true smart vent (like the Flair Smart Vent) is a motorized damper that physically opens and closes to redirect airflow between rooms, usually as part of a multi-vent zoning system. Boosters are cheaper and simpler; motorized smart vents enable whole-home balancing but cost far more per vent.