Simplified use case:
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Client has two warehouses: A and B
Client sells 3 products: A, B and C

Product A is always shipped from Warehouse A > Customer
Product B is always shipped from Warehouse B > Customer
Product C may be shipped from either warehouse

Guidelines for use:
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1. Set the route "Warehouse A: Ship Only" on product A
2. Set the route "Warehouse B: Ship Only" on product B
3. Determine the ship-from location of Product C either using a sale order line route or by simply changing the warehouse of the sale order.
4. Override the ship-from location of products A or B using a sale order line route.

Upon confirmation of the sale order, one delivery order per ship route will be created. With two warehouses, this will be a maximum of 2 delivery orders.

The key background information is:

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0. In general, routes can be set in 4 places. They are, in order of application: Sale Order Line > Product > Product Category > Warehouse

1. There is a warehouse set on the sale order.

2. Warehouses have default shipping routes.

2. You can set a shipping route on a product or a sale order line. This route will override the default warehouse shipping route.

3. Upon confirmation of a sale order, a shipping route will be selected for each order line according to item 0 in this list

4. If no shipping route is set on the product, the default shipping route of the order warehouse will be used.

5. The route on the sale order line supercedes all other routes. 

Therefore when configuring products, note the following:

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1. If you would like to always ship a product from a specific warehouse, set the correct ship only route on the product settings.

2. If you would like to ship a product from whatever warehouse is specified on the order, set no ship route on the product.

3. If it becomes burdensome to set preferred routes on each product, you should be able to set the ship only route on product categories
 

For your reference, my route configuration is as follows:

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On the "Ship Only" route for each warehouse:

1. Remove the "Served Warehouse" from the procurement rule

2. Allow the route to be set on products