From the United States Mint:

Starting today, the United States Mint’s online product catalog will include an additional product status notification to alert customers of product availability. In addition to “Back Order,” “Out of Stock” and “Out of Stock/Remind Me,” we will now include a “Sold Out” notification on the product page.

The “Sold Out” status is based on actual inventory, rather than adjusted net demand of the product, which reflected only orders placed and did not include other factors such as cancellations, credit card approvals, etc. A “Sold Out” status differs from “Out of Stock” in that there is no additional production or cancelled orders that can be used to fulfill orders.

Additional information about other status notifications is below:

Sold Out: When all inventory of products has been depleted, taking into account cancellations and other factors.

Back Order: When products are in “Back Order” status, this means that the product is available for sale, however there is no inventory in stock and a future ship date is displayed on our website. Products in this status can still be ordered online or by calling  Customer Service at 1-800-USA-MINT (872-6468).

Out Of Stock: Products in “Out of Stock” status with no “Remind Me” button are not available for sale. If product becomes available through additional production or cancelled orders, the product will be made available for purchase again.

Out of Stock with “Remind Me” button: When products are in an “Out of Stock” status with a “Remind Me” button, there is no inventory currently available, but additional inventory is likely. Products in this status cannot be ordered, but by clicking the “Remind Me” button you can request to be reminded by email when product becomes available.