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What is an NS record?
NS records name the authoritative nameservers for a zone. They are what a delegation actually is.
The parent zone publishes NS records pointing at your nameservers; your own zone repeats them at its apex. The two lists should agree.
A resolver may ask any of the listed nameservers, so every one of them must hold a complete, current copy of the zone.
If the nameserver name lives inside the domain it serves, the parent also needs glue records — address records supplied by the registry to break the circular lookup.
At CTPFDNS a delegated domain lists all four nameservers, so any one of them can answer on its own.