There are 3 inventory types you will see in Sellbrite: Available, Reserved, and On Hand:
"On Hand" refers to how many of a product you physically have in your warehouse. You can edit the on-hand quantity for your merchant warehouse locations, but not for Amazon FBA.
"Reserved" refers to items that have been ordered, but are still physically in your warehouse. Items are reserved (or set aside) until the order is either Completed (shipped out) or Canceled.
When an order is completed, the reserved quantity will be deducted from the on-hand total. When an order is canceled, the reserved quantity will be allocated back to the on-hand total.
"Available" refers to the quantity that is currently available for customers to purchase (total on-hand minus reserved). This is the quantity that Sellbrite syncs out to the channel. The adjusted available quantity prevents items from overselling.
Here's an example of how the above inventory types apply to an actual customer purchase:
If you have 10 cogs in your warehouse, your On Hand quantity is 10. If you have listed all 10 of these cogs for sale, then your Available quantity is also 10.
When a customer purchases 2 cogs, that 2 quantity will be deducted from your Available quantity (because they have been ordered), and moved to Reserved.
This leaves you with: 8 cogs Available for purchase, 2 cogs Reserved from your order, and 10 cogs On Hand in your warehouse.
After you ship that order of 2 cogs, that quantity will be removed from Reserved because they have been shipped, and 2 will be removed from your On Hand quantity (because they are no longer in your warehouse).
This will leave you with: 8 Available, 0 Reserved, and 8 On Hand.
In Summary, you can think of the inventory types as:
Available = How many can be purchased by customers
Reserved = How many have been ordered, but not shipped out yet
On Hand = How many you have in your warehouse right now