Why are there two air filter slots in my HVAC system?

Magnetic Furnace Filter Cover furnace filter slot cover This patented two-piece system is designed for 1-inch air filters Placing it over the furnace filter slot eliminates air leakage, which improves indoor air furnace filter slot AllergyZone - FilterLock Filter Slot Seal – Seal Cover for Any 1” Furnace Filter, Up To 25” Long Protect Your HVAC System: FilterLock is a patented

pachislo slot machine The FilterLock seals furnace and air conditioning air filter slots to reduce utility bills and improve indoor air quality The Filter Slot Cover eliminates