I think I answered you by email already, but for the sake of others who are wondering I’ll answer here too. ^^

As far as I can tell it means they are out of stock in the online store, but they still have stock left in store branches and such. Once it says ‘sold out’ that generally means it is sold out everywhere including the stores. 'Order’ means they will order it from another store which means domestic shipping takes longer. It also means that by the time you make the order there’s a chance the item is already out of stock when they check for it, but in that case your order will simply be cancelled.

I noticed during the Fairy Marine Chiffon release it was 'sold out’ several times and then changed to 'order’ a couple days later. This is because they still had plenty left in store branches so they were able to transfer them to the online store if required. :)