Nameserver assignment
When you add a domain on a primary (full) or secondary DNS setup, Cloudflare automatically assigns your nameservers. You should then add the assigned nameservers to your registrar and, once Cloudflare can detect they have been placed, your zone is activated.
Each domain’s assigned nameservers may be different than other domains, even if those domains are within the same account.
These nameserver assignments cannot be changed unless you set up custom or vanity nameservers.
For more background on nameserver assignments, refer to our blog.