Any product that has been added to a visitor's Shop shopping cart will be included when determining product availability for that visitor if they choose to add more of that product. However, products currently in another visitor's Shop shopping cart accessing the site at the same time are not included when determining availability.
As an example, Product A has an available inventory of 5. Visitor Y adds a quantity of 3 of Product A to her Shop shopping cart. She decides to add more of Product A. When attempting to do so, she is only allowed to add up to 2 more.
At the same that Visitor Y has 3 of the available 5 inventory of Product A, Visitor Z adds 5 of Product A to his cart and checks out. Visitor Y will still be allowed to check out with the 3 items of Product A in her shopping cart at this time.