Which types of strings are not permitted to be applied for?

Already delegated TLDs
gTLD strings that were applied for in previous gTLD rounds and whose applications are still being processed
Existing Country Code Top-Level Domains (ccTLDs)
Strings currently requested as IDN ccTLDs
All other one- or two-letter ASCII strings
Strings on the Blocked Names list
Strings on the Reserved Names list (unless the applicant is qualified to use the exception process to apply for a limited international IGO-INGO name)
Closed Generics