A nameserver basically responds to a query. Interoperability is an obvious
requirement here. The standard protocol behaviour of different DNS
implementations is expected to be the same.
Requirements for protocol behaviour of DNS implementations is widely documented
in the case of 'common' dns messages. The DNS protocol is over 20 years old and
since its inception, there have been over 40 independent DNS implementations,
while some implementations have over 20 versions.
The methodology used to identify individual nameserver implementations is based
on "borderline" protocol behaviour. The DNS protocol offers a multitude of
message bits, response types, opcodes, classes, query types and label types in a
fashion that makes some mutually exclusive while some are not used in a query
messages at all. Not every implementation offers the full set of features the
DNS protocol set currently has. Some implementations offer features outside the
protocol set, and there are implementations that do not conform to standards.
Also, new features added to - or bugs removed allow for differentiations between
versions of an implementation.