Glass used in doors can be either Laminated Safety Glass or Toughened Glass. If the door frame will only take a glass thickness of less than 6.38mm then Toughened Glass would be used starting at 4mm. While Toughened Glass offers more variety in terms of patterns and workability, Laminated Safety Glass can often be the best bet. In the event of a breakage Toughened Glass will shatter sending small blocks of glass everywhere and while they are relatively harmless you'll have that hole in your door until fixed. Laminated Safety Glass when broken will still stay together offering a barrier to the elements.