The term 'BOUNDARY SECTOR' is composed of two words, 'BOUNDARY' and 'SECTOR'. The word 'BOUNDARY' refers to a line that marks the limits of an area, while the word 'SECTOR' refers to a distinct part or subdivision of a larger whole. Together, the term 'BOUNDARY SECTOR' can be used to describe a specific area within a larger region that is separated by a BOUNDARY line. In the context of a crossword puzzle, the answer to the clue 'ARC' (3 letters) is 'ARC', which stands for 'Area of Responsibility and Control'. This term is used to refer to a specific area of responsibility and control that is bounded by a BOUNDARY line.