What is the meaning of Field_head?

The head formed by upsetting the bucktail of a rivet after it is inserted through the materials to be connected, as opposed to the factory head made when the rivet is manufactured; the field head of a solid rivet is typically pancake-shaped, depending on the shape of the bucking bar or other tool used to upset it. The field head of a blind rivet is typically doughnut-shaped.

Source: wiktionary.org