This is known as a "no show policy". Flights must be flown in sequence. If you fail to check-in to your flight, the entire ticket may be forfeited. For example, if you buy a ticket, Brisbane - Singapore - Brisbane, you cannot just fly on the Singapore to Brisbane flight.
An e-ticket is an electronic version of a paper ticket. This is emailed through following making your booking. If you misplace it, please send a request through to our Customer Care Team for this to be resent to you. For international travel, it is suggested to take a paper copy of your e-ticket and itinerary to present at the check-in counter.
This generates when your booking is made. You will find it on your e-ticket which you will receive via email. You will need it if you want to access the airlines "manage my booking" site.