Vertical sequencer for product order fulfillment

A method comprising:

• providing a multi-level transport system of a product order fulfillment system, each level thereof having a corresponding independent asynchronous level transport system, of mixed cases, separate and distinct from the asynchronous level transport system corresponding to each other level of the multi-level transport system, the asynchronous level transport system defining an array of asynchronous level transport axes, corresponding to the level, and being configured to hold and asynchronously transport at least one case providing transport of mixed cases along the array of asynchronous level transport axes;
• providing a lifting transport system with more than one independent lift axis, each of the more than one independent lift axis being configured to independently hold the at least one case and reciprocate along a lift travel axis independently raising and lowering the at least one case, the more than one independent lift axis each being communicably coupled to a common lift transport output through which each of the more than one independent lift axis commonly output the mixed cases from lifting transport system;
• and
• providing an infeed interface communicably coupling the multi-level transport system with each of the more than one independent lift axis, the infeed interface comprising different infeed stations distributed at each asynchronous level transport system for each of the more than one independent lift axis so that each of the more than one independent lift axis has a different corresponding infeed station at each asynchronous level transport system through which mixed cases feed from the multi-level transport system to each of the more than one independent lift axis;
• effecting output, with the more than one independent lift axis, of mixed cases substantially continuously through the common lift transport output in a predetermined case out ordered sequence decoupled from an available sequence of mixed cases from and created by the multi-level transport system at the infeed interface and that feed the more than one independent lift axis through the infeed interface.