We have the following in Web IDL:

interface NDEFMessage {
  constructor(NDEFMessageInit messageInit);
  // ...
};

interface NDEFRecord {
  constructor(NDEFRecordInit recordInit);
  // ...
};

interface NDEFReadingEvent : Event {
  constructor(DOMString type, NDEFReadingEventInit readingEventInitDict);
  /// ...
};

• There is nothing in the spec saying what NDEFMessage's constructor is actually supposed to do and how it is supposed to use messageInit.
• I can see some explanation about NDEFRecord and NDEFRecordInit here, but some of the description of what to do with some of recordInit's members is in this section and the rest spread across the spec, which makes it quite confusing to follow.
• There is a little bit of explanation about NDEFReadingEvent, but the actual steps describing how readingEventInitDict.message leads to the creation of a new NDEFMessage are not there.