HEPA-13 is medical-grade purification technology that traps 99.97% of particles up to 0.3 micron, and 99.9% of particles up to 0.1 micron in diameter.
Filters with a lower HEPA number (e.g. HEPA-11) aren't as good at trapping particles, which means there are more contaminants in the air you're breathing at home.