Air filter grades describe what a filter catches, and the two families behave completely differently. Coarse filters, G2 through G4, are the coarse mesh of the system: they catch dust, lint, insects and construction debris so the coils, fans and fine filters downstream stay clean. Fine filters, F5 through F9, do the invisible work: pollen, smoke particles, bacteria-carrying dust, the matter that matters in hospitals and pharma.
The mistake we see most often is a fine filter asked to do a coarse filter's job. An F8 bag filter facing raw outdoor air loads up in weeks, pressure drop climbs, the fan works harder, and the power bill quietly pays for the shortcut. Run a G4 panel in front of it and the same bag filter lasts a full season. Filtration is a supply chain, and every stage protects the next.
Our rule of thumb for Indian sites: residential and commercial comfort air is served by G4 plus F7. Healthcare, laboratories and electronics step up to F8 or F9. Kitchens run washable aluminium mesh pre-filters before their bank. And whatever the grade, size the filter to the housing and track pressure drop, because a filter that is 'fine' on paper and clogged on site is filtering nothing but your energy efficiency.