Few employees were aware of the existence of the backup server until the outage.

直到系統當機前，很少員工知道備用伺服器的存在。

The auditor questioned the existence of several invoices listed in the quarterly report.

稽核人員對季報中列出的幾張發票是否真的存在提出質疑。

Please confirm the existence of a valid visa before booking the client's flight.

在幫客戶訂機票之前，請先確認他持有有效簽證。