In the product setup screen of the cart, the "Booking units" and "quantity on hand" fields control how your inventory is managed and how user purchases decrement inventory days being available (green) or unavailable (red) in the calendar on the product details page.
For example, if you configure:
Booking Units = Daily
Qty On Hand = 100
Configures the cart to allow up to 100 quantity purchases per day before the day is marked red on the product details calendar.
Let me know if that answers your question.