This phrase is known from post-WWII Germany, written on boundary markers between the various sectors. Does anyone know if this was the first usage, or if the phrase was used in some earlier context? It may not have been originally in English, it's worth bearing in mind.
Sector is a mathematical word meaning a portion of a circle bounded by two radii and an arc. This is the meaning used by the "X sector" signage in occupied Berlin, as the various Allied occupation zones approximate sectors about the city center:
An earlier usage of "sector" to describe geographical areas would be the sector model of urban planning (1930s), where different types of land use are constrained to sectors radiating from a Central Business District. The advantage of this model is that the city can grow outwards in the same layout.
"Sector" has also been used metaphorically, as in "the sectors of the Hospitality industry". I believe this usage stems from the popularity of the pie chart, which dates to 1801.