To be capable of supporting any language content, a domain name must have active services. Active name servers enable a domain supporting active services, such as a website or email.
A high percentage of active name servers can be an indicator of quality in a TLD and of stability, as domain names which are actively in use are more likely to be renewed. Where a TLD has a low percentage of active name servers, the individual domains comprised in that TLD could be less likely to renew.
The purpose of this analysis is not to pass any judgement on the usage rates within a particular TLD, but to eliminate from further analysis those domains categorized as inactive.