Most of the time it means the product is out of print, and won't be available again. There are some exceptions, though:
- The RPG-line of products (Core Rulebook, APG, UM, UC, Bestiaries, etc) will be reprinted once they go out of print, and will thus become available again.
- If a product has sold out very quickly, it might be reprinted. As far as I can remember, this has only happened with the Adventurer's Armory.
- Theoretically, one of the distributors/retailers Paizo has sent the book to might at some point in the future return copies they haven't sold.
- And finally, it could be an error in the system, but that's unlikely, and if it happens it would get fixed quickly.
As far as I can remember, this has only happened with the Adventurer's Armory.
For those that find this interesting, The Skinsaw Murders got a reprint too.
- If a product has sold out very quickly, it might be reprinted. As far as I can remember, this has only happened with the Adventurer's Armory.
Distant Worlds also got a reprint, IIRC.
Unavailable means that we are not taking orders for a given product at this time.
For Paizo products, it usually means the product is out of print and we are not currently planning to print more. (If we are planning to print more, the product will usually be a backorder.)
For non-Paizo products, which we order in from distribution, it means that we do not know if or when we will be able to get more. Once again, if we are confident we will be able to order it in the future, the product will be a backorder. Sometimes it means there is a miscommunication somewhere with the distributor or the manufacturer and more are on the way, but we don't know that. Sometimes things just change and an out-of-print product unexpectedly gets reprinted. However, usually this means the product is permanently out of print.
(You will note I say 'usually' a lot in this description. There are always exceptions and plain old (human or electronic) error in things like this.)