Density of the clay body is the number one indicator of quality. The less air pockets in the clay the denser, hence stronger the tile will be.There are 4 different grades, porcelain tile being the highest grade of ceramic, vitreous, semi-vitreous and non-vitreous. Porcelain tile is defined by having a water absorption rate of less than one half percent(0.5%) or less. These tiles are very unlikely to chip or crack if something heavy is dropped on them. Vitreous tiles absorb less than 3% of their weight in water and are somewhat unlikely to chip or crack. Semi-vitreous tiles absorb more that 3% but less than 7% of their weight and could chip or crack, and finally non-vitreous tiles absorb more than 7% of their weight in water and quite likely will crack when something heavy is dropped on them. These are the type of tiles you may see from time to time being cleared out at a big box store.
Some basic product knowledge
Close up side view of clay tile products and how air pockets determine density
Non-vitreous tile absorbs more than 7% of its weight in water due to its many air pockets. Semi-vitreous tile absorb between 3% and 7% of its weight in water. Vitreous tile absorb between 3% and 0.5% of its weight in water.
Impervious tile absorb less than 0.5% of its weight in water and because of this is considered frost proof and can be used outdoors.
The water absorption test is used only to determine quality of floor tile!