Maybe I'm wrong but AFAIK this is used for creating the appstack the first time.
After installing and sealing the appstack this line should be removed as it is no longer writable. I think the phrase being used might be a bit confusing as it seems to be a writable volume. But you can merely write and not only read the disk (hence it is writable) until you seal it.
its set to writeable while your provisioning it I beleive, once you finish the appstack it would change to the correct type.