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UltraCart tracks the inventory allocation by taking the configured quantity in stock, then deducts from the quantity (qty) in stock this way:
Allocated to CSSC (Shopping Cart) - This allocation is a temporary allocation to active shopping cat cart sessions.
Allocated to PO (Placed Order) - This allocation is to placed orders that have not yet been to be marked as shipped (reached the "completed" stage)
Available to Allocate - This represents the QTY in Stock minu minus the allocated inventory for the SC and PO. This is what is av available at the present time.