We have up to six separate R/3 instants. Can STANDARD APO functionality be used for generating a constraint distribution requirements planning that take into considerations of capacity and availability of inventory across all the different resources (machines, inventory etc) in the different R/3 box?
Yes, standard Supply Network Planning (SNP) within APO can achieve this, although a lot of thought needs to go into the design and set-up of the system and data. The main aspect of achieving the single distribution plan would be to have one APO instance that connects with your six separate R/3 systems. Within SNP, one option would be to use Capable to Match (CTM) which creates a feasible multi-site solution based on the available stock, production capacities and transportation capabilities.