Aluminum covers question

[Pars]

I see in the diyaudio store that there are options to order chassis with either steel or aluminum covers. For a 1U Slimline 230mm chassis, if I choose aluminum covers, I also see a button labeled Slimline: Substitute alternative covers

Doesn't the aluminum covers option already cover this? Or do you also have to choose vented/blank, etc.?

Thanks for the clarification.