In the old days signies had a choice of 9mm or 12mm thickness of Signply. Manufacturing changes of about 12 years ago change this to 8mm, 10mm as well as 12mm. Most signwritters have taken the soft option by choosing 10mm as a favoured thickness in lieu of the 12mm. They use 12mm when that extra bit of strength is required – shop fascias etc.
However let us not forget about the 8mm. Not only is it 1mm thinner than the old favoured 9mm but has five veneer thickness, which gives it great strength, for its thickness. The old 9mm was only a 3 ply.
8mm suitable anywhere that has a backing, gable ends to factories, walls, fences etc. Cheaper, lightweight and strong.