Now product availability is totally controlled by inventory items.
Suppose you have a product named "polo shirt" which you want to rent out. You have 6 of these with 2 small, 2 medium and 2 large shirts available. So you need to create 3 inventories and you can give them the name "small", "medium" and "large" or any other of your liking. While you create the inventories you add the categories, resources, pick up location, drop off location etc. as per your requirement and publish the inventories. So now you will have inventories like below
Now you need to attach the inventory with the "polo shirt"
This will now mean that you have 6 polo shirts of 3 different sizes and a user can book .
Then availability is controlled with these child posts of the inventory items.