Yes it is correct. Like most countries, Thailand law is that a foreigner should carry at least a copy of their passport showing the details page, the latest entry visa stamp and the white departure card that is placed in your passport when you enter Thailand. It can also be for your own safety. I always advise having a copy of your passport because if the eventuality arose were you lost your original passport, it will save you a lot of heartache at your embassy when applying for a new one. If you call in at our office, for a small fee we can produce a credit card sized laminated copy of your passport which you can carry with you at all times with ease.