How to calculate the layers id in manifest.json in image.tar and what does it mean?

Docker version 1.12
Docker registry version 2
Operating system ubuntu14.04

After using docker save command to export local image myubuntu,I got myubuntu.tar. After decompressing myubuntu.tar, I got the manifest.json like figure 1.figure 1 manifest.json. I cannnot understand the meaning of layers key of this file. I compared this manifest.json with manifest I got from the registry v2 api(I have pushed this image to my own registry service). The manifest I got from registry is like manifest from registry. Obviously, layers in manifest.json and manifest use different IDs. The id in layers of manifest.json is not blobsum which means the sha256 digest of compressed layer data. And it’s not diffid which means the sha256 digest of uncompressed layer data(I have checked the configure json of this image.).

So, what does layers of manifest.json mean and how to calculate it?
Thank you.