Originally Posted by c1223

On the "Booking Summary" page where it asks you to "review you flights" if you scroll down it'll say "Your fare conditions". Open that up and have a look. It'll say something along the lines of:

Originally Posted by Finland Station

I would recommend calling BA before you book this ticket. There are one-way fares (booked in D class) SYD-SIN that have a cancellation penalty of AU$150 (which is pretty low). It might be worth investigating all the options before you select the one that is best for you. BA.com is a bit of a blunt tool for making a more sophisticated choice.

Originally Posted by frankvb

Since the fully flexible fare could be rather astronomical, doesn't it make more sense to hold off on being the ticket until you are certain? Though certain (lower) fares could indeed be flexible just with a relatively small cancellation/refund penalty. You could try https://oldmatrix.itasoftware.com/ to get a better idea about the fare rules.