It's actually a flag on the cd when it's made of if it's a cd-r or rw. The software can choose to ignore this and use it as rw. It's not advisable however since rw cd's are higher quality than just plain cd-r's.

ie. you might get 1000 burns out of a single cd-rw but only 5 out of a cd-r used as an rw.

There is other software that will ignore the r/rw flag and use it how you want... but it's really not recommended. I've done it and gotten a few burns out of a cd-r, but have cd-rws that have been burned hundreds of times over years. The cd-r's sometimes get 2 burns, sometimes 10. You never know.