noun
Used for items or personnel kept as a backup or substitute. Often appears in compound nouns like 補用品 (spare parts/supplies) or 補用要員 (reserve personnel).
補用のタイヤをトランクに積んでおく。
I keep a spare tire in the trunk.
この部品は補用品として倉庫に保管されている。
This part is stored in the warehouse as a spare.
Compound of 補 (supplement, compensate) and 用 (use, purpose). The exact historical derivation is uncertain, but the meaning follows naturally from the kanji components.