He defended his lawful rights and interests through legal channels.
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A boilerplate phrase in Chinese law and policy writing — almost always paired with 维护 (wéihù, safeguard) or 保护 (bǎohù, protect). 权益 fuses 权利 (quánlì, rights) and 利益 (lìyì, interests), neatly covering both legal entitlements and material benefits.