What is "Defense Area" on a VFR Sectional Chart?
FAR 99.3 defines a Defense Area as an area in which air traffic can be controlled in the interest of national security. In other words, non-ADIZ airspace in which SCATANA or a similar plan can be implemented. In other words, all domestic airspace. It used to be clear that all airspace was Defense Area until FAR 99 got rewritten a couple of times in 2001-2004 when they were trying to figure out if the DC SFRA was an ADIZ. Now, the FAR that explicitly says that all non-ADIZ airspace is Defense Area is gone (to who knows where).

Nonetheless, all airspace that isn't an ADIZ is Defense Area, from sea to shining sea. 

So why is it depicted on the southern border? To clarify that the airspace is not an ADIZ.



Take a look at this piece of the San Diego TAC. Notice how out in the ocean, the ADIZ is an actual 2-dimensional region. It's depicted with solid magenta lines with dots protecting the inside of the ADIZ. But at the beach, it devolves into a 1-dimensional line. But it's still depicted as the magenta line with dots. Look right there at the beach what has to happen, how the dots have to jump over the line. Thing is, since the ADIZ is a line there, the dots are actually outside of it, they can't protect the inside of it (since there is no inside to protect) and it looks like maybe all of California is an ADIZ, when it isn't. So the words DEFENSE AREA are printed there across the border from CONTIGUOUS US ADIZ to emphasize that the dots aren't actually protecting an ADIZ area; the ADIZ is too narrow to be correctly depicted.