Systems and methods are provided for automatically determining substitution chains that are valid during a rules-based ATP check for an order item. In one implementation, a system is provided that includes 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. The system also includes an execution memory storing software and a buffer for temporarily storing data. A processor coupled to the interface and the execution memory may carry out an availability check in which a replacement chain describing replacements is stored in the buffer. The replacement chain may be determined by one or more of rules defining substitutions.