https://reference.opcfoundation.org/Core/Part6/v105/docs/5.4.1 first says:
"The JSON format is defined in RFC 8259. It is partially self-describing because each field has a name encoded in addition to the value..."
which can make one think 'field' later mean a JSON field, but then it says in a later paragraph
"The CompactEncoding omits all fields with a value equal to the default value for the type. The VerboseEncoding includes all fields."

It is possible to interpret that to mean ALL JSON fields. However, based on the PubSub new JSON formats Plugfest, I think here actually 'Structure fields' are intended instead. Because e.g. DataValue (https://reference.opcfoundation.org/Core/Part6/v105/docs/5.4.2.18) e.g. for the status field it says "Not encoded if the value is Good (0).", which would have conflicted the wording if it would have been about JSON fields (i.e. do you or not put it in Verbose when it is 0). If the wording is changed to something like:
"The CompactEncoding omits all Structure fields with a field value equal to the default value for the field type. The VerboseEncoding includes all Structure fields."
it shouldn't be ambiguous anymore.