We have tried telling the ATUM team multiple times about this. However, they are a bit rigid on this matter.
It is a simple deduction from a business point of view:
Ship the product that is deliverable fastest (FIRST) -- this is achieved by Region Restriction
Use a fallback method, example: Expiring First
Consider this case:
Inventory = 5, Location = India, Expiry = 01 st April
Inventory = 5, Location = USA, Expiry = 02 nd April
A customer from US orders the product. We have set the sorting method to Expiring first.
Atum will assign him India's inventory -- which is absurd.
The plugin needs to have 2 sorting methods, first obviously based on Region Restriction and if this method returns false (ie. product is out of stock) a fallback sorting should kick in (expiry first, fifo,lifo) etc.
Even Amazon follows this method. Customers of today want faster deliveries.
If we use only region restriction to ensure faster delivery, then the product is always out of stock for them inspite of it being in stock with the business.
TWO SORTING methods resolve this equation beautifully.