If any 3rd-level names underneath some second-level name are already registered, whether to you or anybody else, you can't register the second-level name, since that is kept by the registry in order to service registrations (and e-mail forwarding) for the underlying third-level names. If no 3rd-level names are registered under some 2nd-level name, then that name is available for registration, and if you get it, nobody would be able to register 3rd-level names beneath it from then on, since you'd own the name and would have control (on your own, or your hosting provider's, server) of any subdomains that might be created within it as well as e-mail addresses.
It really works just like any other domain, other than the confusing issue of the third-level registrations with 2nd-level mail forwarding; if you own xyz.com or xyz.name, you can create unlimited subdomains within it like abc.xyz.com or abc.xyz.name. But if abc.xyz.name were already registered as a third-level name, then xyz.name would be reserved by the registry and you couldn't register xyz.name yourself, but you could register other third-level names under it like def.xyz.name, and if you owned that you could then create fourth-level subdomains like abc.def.xyz.name.
More info on subdomains:
http://domains.dan.info/structure/subdomains.html
Info on new TLDs:
http://domains.dan.info/structure/new-tlds.html
It really works just like any other domain, other than the confusing issue of the third-level registrations with 2nd-level mail forwarding; if you own xyz.com or xyz.name, you can create unlimited subdomains within it like abc.xyz.com or abc.xyz.name. But if abc.xyz.name were already registered as a third-level name, then xyz.name would be reserved by the registry and you couldn't register xyz.name yourself, but you could register other third-level names under it like def.xyz.name, and if you owned that you could then create fourth-level subdomains like abc.def.xyz.name.
More info on subdomains:
http://domains.dan.info/structure/subdomains.html
Info on new TLDs:
http://domains.dan.info/structure/new-tlds.html