[Btrfs-devel] Data=ordered - what for?

[sftf]

Hi!
Till now I thought btrfs does not use journaling at all, but utilize COW approach for
operations atomicity.
But, when I have seen in "Btrfs Timeline":
July 31, 2008: Data=ordered mode support, preventing null bytes in a file after a crash.
I have begun to doubt - whether I correctly understand atomicity and transactions concepts
of btrfs.

Please, can you clear these doubts?
Is btrfs uses any kind of journaling and what for "Data=ordered"?
I guess, may be it for metadata-data write order but without journal involving, isn't it?
Thanks!