12"x 8" (Duct Opening Size) 2-Way Stamped Face Steel Ceiling/sidewall Air Supply Register - Vent Cover - Actual Outside Dimension 13.75" X 9.75" it refers to the size

it refers to the size of the "tongue"  so that it will fit in a 8"x8" hole

The Listed size 10"x 6" is for inside duct opening measurement

Acting as a reliable safeguard

Durable Construction: Made of galvanized steel for long-lasting corrosion resistance

Do not use outside measurement for size selection or it would be too bit to fit

12"x 8" (Duct Opening Size) 2-Way Stamped Face Steel Ceiling/sidewall Air Supply Register - Vent Cover - Actual Outside Dimension 13.75" X 9.75" it refers to the sizeDurable Construction: This air supply register is made of sturdy stamped steel for long lasting use and reliable performance. Versatile Application: Designed for ceiling or sidewall installation, this vent cover can be used for air supply in various residential and commercial settings. Efficient Airflow: With a 12" x 8" duct opening size, this register allows for optimal airflow distribution from your HVAC system. Easy Installation: The actual outside