You don’t say how far out this is?
Normally, you can select the cabin, unless: It’s already sold out or if you’re closer in, say, just after final payment, they could be working upgrades. Many variables.
Or, if you are just before final payment, you might wait till after final payment and recheck, in case there were cancellations in your category and you can then secure a cabin you want.
Aft balconies are very popular. I have been able to pick mine every time, except once, I did a guarantee. It worked out great. YMMV