The term DEPEND ADD-ON can be broken down into two crossword solutions: ENCE (4 letters) and ENCY (4 letters). ENCE is an abbreviation for the word 'ENCEPHALON, ' which is the medical term for the brain. ENCY is an abbreviation for the word 'encyclopedia, ' which is a reference book containing information on a wide variety of topics. Both words are related to the term DEPEND ADD-ON, as they both refer to knowledge and understanding.