Tile Ratings refer to a tiles hardness.  All tiles feel hard, but some types of tiles are actually harder than others. Tiles are rated by a series of standardized tests. The tests evaluate a tile's relative hardness (the Moh scale), its ability to stand up to wear and the percentage of water absorbed.

The Porcelain Enamel Institute hardness ratings are:

• Group I - Light Traffic: residential bathroom floors where bare or stocking feet are the norm.
  
  
• Group II - Medium Traffic: home interiors where little abrasion occurs. Don't use in kitchens or entries.
  
  
• Group III - Medium-Heavy Traffic: any home interior.
  
  
• Group IV - Heavy Traffic: homes or light to medium commercial areas.
  
  
• Group V- Extra Heavy Traffic: use it anywhere.

These ratings are important, but don't get too bogged down in analysis — they serve to help you find the right tile for your application.