Take the following example document:

{
   "_id": "abc123",
   "name": "Testitem",
   "item": {
      "color": "blue",
      "size": 4,
      "price": 10
   }
}

If the document is updated using:

{
   "$set": {
      "item.color": "red",
      "item.size": 5
   }
}

Then a change stream listening to updates will receive a response like this:

{
   "updateDescription": {
      "updatedFields": {
         "item.color": "red",
         "item.size": 5
      }
   }
}

Because the updated fields are written to updateDescription.updatedFields with a dot as a separator, it is impossible to query the updatedFields (see SERVER-30575) for example to only listen to updates if the "item.color" property changes.