The word 备用 is formed by combining two characters: 备 (bèi), which carries the core meaning of "to prepare" or "to have ready", and 用 (yòng), which means "to use". The formation conceptually merges these ideas into a single unit meaning "prepared for use" or "reserved for use". It describes something that is kept in a state of readiness, not for immediate or primary application, but to be available and employed if and when the need arises. Thus, the entire compound naturally conveys the idea of a backup or reserve.