It is important to note that there may be sub-assembly functions, and that move functions may be applied to a single SKU, to a batch of the same SKU, or to a batch of different SKUs. For example, if items are order picked to a carton from bin shelving, and the carton is transported to a mixed pallet build station, the “order pick to carton” constitutes a sub-assembly function. These kinds of variations are easily accommodated, as are reserve storage with forward picking systems.
Source: wiktionary