The key part is where it says: "These items are available only on the Mac where you created them: Faces tiles and Faces data".

Essentially, Apple Photos is the next version of iPhoto, but with much better syncing added. However, they haven't yet added sync for everything. I would like to be able to start a book project at home and finish it at my work computer, but because, like faces, books are not yet synced, this isn't possible.

Hopefully, they will syncing the rest of the data in the next release or two.

The key part is where it says: "These items are available only on the Mac where you created them: Faces tiles and Faces data".

Essentially, Apple Photos is the next version of iPhoto, but with much better syncing added. However, they haven't yet added sync for everything. I would like to be able to start a book project at home and finish it at my work computer, but because, like faces, books are not yet synced, this isn't possible.

Hopefully, they will start syncing the rest of the data in the next release or two.