Polished porcelain tiles are tiles made entirely from porcelain that have had their surfaces polished rather than having a glaze added.
Many tiles, including some that are made from porcelain, have a layer of glass added to the surface (we call this process glazing) that protects the biscuit (or body) of the tile from staining and damage.
As porcelain is extremely strong and durable as well as being virtually non-porous, meaning that it doesn't absorb liquids, porcelain tiles don't need a layer of glaze to protect them. The advantage of this is that the surface of the tile can be polished to any level of shininess from a light 'lappato' finish right up to the high shine we usually think of when we talk about 'polished porcelain.'
Although the tiles are non-porous, they can contain microscopic bubbles under their surface. During polishing these can get opened up and effectively become tiny cups on the surface of the tile.
Often when polished porcelain appears stained, all that has happened is that a liquid has been spilled and is filling these cups. As regular cleaning doesn't wash the liquid out of the cups, it can seem that the tile is stained. In fact, it can be removed with the right kind of washing, but can also be avoided altogether with correct sealing of the tile.
Polished porcelain tiles should be sealed with a special sealer when they are laid. Instead of coating the surface, this sealer fills the microscopic cups, not allowing anything else to get in.
It’s also important to note that when polished porcelain comes into contact with chlorinated water and acids that it will attract stubborn stains and the surface can become brittle.