dd question (from man page)

[jd1008]

Man page says:
...

        fdatasync
               physically write output file data before finishing

        fsync  likewise, but also write metadata


There is no explanation about this, as dd is supposed to be agnostic
about the type of the data.

If a disk is being dd'ed  out to a file and the disk is mounted,
(which is a BAAAAD thing to do), then even there, dd is agnostic
about FS data and FS metadata.

So, what data and metadata is the manpage talking about?