Industry criteria dictate that tile shall not exceed 5% water absorption to be recognized as frost resistant.  To be recognized as Porcelain tile, the water absorption rate cannot exceed 0.50 % (impervious).  Wall tile must not exceed 20% water absorption and is not frost resistant.

Both water absorption and Coefficient of Friction (C.O.F.) ratings should be considered for wet or exterior installations.  The higher the C.O.F. rating, the more slip resistant the tile.  See individual tile series for C.O.F. ratings and ADA compliance.

ANSI A108 sets the following bonding mortar contact requirements for Exterior or Shower installations:  

A-4.3.3.3.2   Thoroughly beat all tile or tile assemblies into place to obtain maximum contact of bonding mortar on the back of each tile, or back of each tile and back mounting material.  Average uniform contact area shall be not less than 80 percent except on exterior or shower installations where contact area shall be 95% when no less than three tiles or tile assemblies are removed for inspection.  The 80% or 95% coverage shall be sufficiently distributed to give full support to the tile with particular attention to this support under all corners of the tile. 

It is imperative that these guidelines for mortar coverage in conjunction with industry-approved installation methods outlined in the TCNA Handbook for Ceramic Tile Installation (particularly relating to proper drainage and slope) plus job-specific setting materials and grout be used to prevent freeze-thaw damage on exterior installations.