He didn't know the particulars and only spoke in general terms.
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Slightly archaic / literary. You'll meet this word mostly in classical novels — 《水浒传》 (Outlaws of the Marsh), 《三国演义》 (Romance of the Three Kingdoms), and Ming-Qing fiction. Modern equivalents: 详情 (xiángqíng) or 细节 (xìjié).
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备 (bèi) = 'complete / fully equipped' (as in 备齐 bèiqí, 'ready in full'); 细 (xì) = 'fine / detailed'. Literally 'fully detailed' — the comprehensive fine-grained account, not the rough summary.
