Inventory Planning, why does it not count a transfer order as supply?

Good information.  I would plan with DRP for everything because it processes all warehouses at the same time particularly when a warehouse is replenished by transfer , handles stock and non-stock kits, generates exception messages to monitor existing supply orders, allows for forecast, uses days of supply to consolidate planned orders (think of a situation where multiple satellite warehouse are sourced from a central warehouse and you want to create one supply order for the central warehouse), and it will minimize inventory investment because planning is time phased.  You can keep using IR to calculate reorder point and safety stock., after all it’s free - but it only looks at invoices and not dependent demand.

Thank you ​@dgodsill97 - we will test ‘using IR to calculate reorder point and safety stock’ even though the Purchase Source item is set to DRP as Planning Method. We didn’t know that was an option.

Items set to DRP do not appear in the Calculate Replen Parameters screen

Well that’s just ducky!  Please enter a support case.  I don’t know what they were thinking when they decided that.  There no reason to exclude the calculate and prepare based on the inventory planning method.