Two distinct object instances, with or without identical fields, might have the same identity and hence represent the same object
I don’t understand this line :/
I took it to mean something like: I fetch an object from a remote source. I modify a field but not any identifier or field critical to identity. I now pass that object back to the source. In some cases (e.g. an update API) this is totally normal, but in others it can be a bit odd that two objects with the same identity are not identical.