1.
copy; duplicate; record kept for reference
A copy or duplicate of a document, receipt, or record kept for one's own reference. Common in business and administrative contexts.
2.
reserve; substitute; backup
A person or thing held in reserve, ready to step in when needed. Used in sports for substitute players and in other contexts for backups.
Usage: 控え is the noun form of 控える (to refrain; to note down; to wait). As a noun, its two main meanings are (1) a duplicate/copy kept for reference and (2) a reserve/backup.