Stock PO add-on gives you a functionality of having 2 type of vendors: stock and distributor.
Distributor process orders, i.e. ship items for specific customers. But physically does not possess products in warehouse.
Warehouse located at stock vendor location which does not process individual customer orders. But prepare items that distributor need in order to process customer orders.
Here is a scenario how this all happens:

1. customer1 place order, customer2 place order …
2. system generate normal POs for distributor vendor according to udropship configuration
3. once a day automatically or manually by admin area request system generate stock po which aggregates items from distributor's POs create record in system. Stock vendor see stock POs in vendor portal, add tracking number, change status. It's possible to send email notification once system generate stock po
4. stock vendor physically send items from stock po to distributor who then process customer orders one by one

At the moment system manage one to many connection between distributor and stock vendor, i.e. stock vendor can have only one distributor, and distributor can have multiple stock vendors

First of all you need to associate products with stock vendor, i.e. in product edit > dropship vendor attribute refer stock vendor. Then in stock vendor edit > vendor info > Distributor select which vendor will process customer orders. System will generate POs for distributor vendor and Stock POs for stock vendor.
By default system generate stock POs once a day. Though it's possible to manually generate stock POs. You can do this from sales > dropship > stock POs > generate stock po

In admin area you can view stock po records, change status, add tracking numbers, view packages tracking information (if system carrier support tracking). Also at almost all locations you have information about distributor (dropship vendor) and stock vendor associated with POs and stock POs.

Stock vendor at his vendor portal can view stock pos information, add tracking numbers, change status of stock po and see distributor location where he need to send items from stock pos. In addition stock vendor receive notifications about new stock pos that system generate