If you are looking for technical "deep post" advice related to construction (perhaps a specific brand or local jargon), general standards for setting posts deep include:
A standard guideline is to bury posts at least one-third of their total height. For example, a 6-foot post would need about 2 feet of depth. If you are looking for technical "deep post"
Usually require more stability, often 42–48 inches deep. or even 4 feet
In many areas, posts must be dug to at least 3 feet 6 inches, or even 4 feet, to reach below the frost line and prevent shifting. Specific Fence Types: Standard Line Posts: Typically require 36 inches of depth. If you are looking for technical "deep post"