Files in netid format are rare. One lives in the YP map netid.byname. The format is rather simple. Each row consists of two items: a key and a value. When created by mknetid(8) there are three types of records.
The first type is information about which GIDs a UID has:
.Sm off unix.<uid@<yp-domain>>
.Sm off <uid:<gid,<gid>>>
The second type contains information about hosts:
.Sm off unix.<hostname@<yp-domain>>
.Sm off 0:<hostname>
The third type refers to records from a netid file other than the two types above.