This is actually not a super-easy question and often depends upon your existing setup. That said, I would urge you to consider using PO25 to manage this as well, but it may not work for you (depends on how other things are setup. Read on for more info
If you were implementing Lawson from the ground up, I would urge you to consider this data model. Since you are already live, you can review this and then look at your own data to see if your data model aligns.
1. Only setup true perpetually tracked items in IC12 (tracking flag will be set to YES for all IC12 items);
2. Only setup true par items in IC81. Naturally, you would also need to setup the SOURCE for each item so Lawson knows whether these are storeroom items or purchased direct from the supplier
3. Create and maintain PO25s for ALL ITEM MASTER items
4. Setup each req loc with a FROM LOCATION = the local storeroom that services their facility (or if you do not have a storeroom at a given facility then you simply setup the FROM LOCATION = main dock (IC02).
With this data model in place, reqs placed at the main hospital will automatically be routed to the storeroom when the item is active in IC12 and will automatically be routed to the supplier when they are not active in IC12.
With this model, you can also support one-offs where different facilities and/or departments purchase the same supply from multiple suppliers. For non-stock items, this is accomplished by setting up multiple PO25s and using the PARTICIPANTS screen to assign participants where necessary. (You only need to setup participants for the PO25 that is used by a subset of the enterprise). For par and perpetual items you can route items to any supplier you like using the ITEM SOURCE Tabs within IC12/IC81 (but you also need PO25s to ensure contract pricing defaults)
In your example, you don't need two PO25s unless the ambulatory clinics order from a different supplier than the rest of the Enterprise.
Hope this helps.