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During airlines check in, including Singapore, at a computerised active kiosk or staffed desk, you are asked to scan or present a valid travel document, e.g., passport, any visa(s) required for entry to destination(s), and, in some cases, reservation detail (scan or enter barcode, e-ticket or booking reference number).
Some airlines may ask to see the credit card with which tickets were purchased, to avert fraudulent or unauthorised use. Failure to do so can result in a refusal to board you.