1. A system for automatically determining substitution chains that are valid during a rules-based ATP check for an order item, the system comprising: an interface for interfacing with a plurality of data storage devices in at least one of which a plurality of stock items are stored as data; an execution memory storing software; a buffer for temporarily storing the data; and a processor coupled to the interface and the execution memory, the processor executing the software to carry out an availability check to identify a replacement chain describing a replacement product for at least one of the plurality of stock items, wherein the availability check references an order due list database element comprising one or more tables having structures based at least on a profile that defines an item processing sequence, the order due list database element providing an index to pre-select and sort potential replacement products, the availability check determining a location of the replacement product, an available quantity of the replacement product, and a date of delivery of the replacement product, and wherein the replacement chain is determined by one or more rules defining substitutions.