The Fence Post! Different depths for different Post types!

Augered Post Hole!

        Fences are made of many different materials, some are of solid panels that catch the wind, and some allow the wind to flow through, so it must allow that the depths of their posts are all set to different depths also. I have lived and operated a Fence Company in 2 different places. Northern Tennessee and Southern Indiana. The "Mean" Frost or Freeze line was 3 inches in the Volunteer State and 6 inches up here in Evansville, Indiana. The only Building code that applies to installing Fence Posts state that 1/3 of the above ground length of the post for a 6' fence must be in the ground. That's it! That's all it says! Lucky for you, w have been digging holes and setting posts forever. The statement above about 1/3 of the post just delineates that for a 6 foot tall Wood fence you need to have 2 Feet in the ground, - not in concrete necessarily, just set in the ground. For 8' Wood Fences we do 32"inches. 
Setting Posts, in general: 
        Holes may be dug be any method, or tool. The best tools to get the results that are required is the Auger or a set of post hole diggers.