You've got two unrelated questions in there (the second one is flat inorrect).

To the first question, you can't really "select" it online, although you can try and force it to different fare classes by selecting different types of fares (Flex, etc.). Still, you will only find out what subfareclass it is when you click all the way through as CXFlyerBoy says. If you must insist on a certain sub fareclass, then you have to do it via the call center. I am less familiar with Y, but I know in J you can realistically get enough optionality via the options (Flex is always J or C, and the few below it generally bucket into D, P or I). I understand from here economy has more and is a bit more complex but there may be some logic to the wording.

The second question, no it's definitely not random.