They are generally designated as 1.01, 1.25 or 1.33, referring to the weight in pounds per lineal foot.
When driving the post with a post pounder the white top paint is a visual means to ensure the user doesn’t raise the pounder too high while pounding.
In areas (such as the British Isles) where treated timber is relatively inexpensive, wooden fence-posts are used and steel ones are unusual for agricultural purposes.
In the British Isles steel posts are however often used for fencing into solid rock.
In this case a hole is drilled into the rock, and the post is fixed using cement or epoxy.