Here is an example:
domain: 8.8.8.e164.arpa
descr: Example domain for the reserved prefix +888
[...]
nserver: ns1.8.8.8.e164.arpa 192.0.2.1
nserver: ns2.8.8.8.e164.arpa 192.0.2.129
nserver: ns1.example.net
nserver: ns2.example.net
[...]
In this example, the first two "nserver:" attributes contain one glue IP address. The last two "nserver:" attributes do not need glue, as they are in a different domain – example.net.