Jaideep... that sounds interesting. If I setup the replentishment with zero quantities, and select a route, that could do it... But in the product template page, in the Inventory tab, do I keep all routes without checkmarks?

Yes on the product inventory tab you would select ‘all‘ applicable routes, while on replenishment you would select the ‘preferred’ route along with trigger, min, max quantities for each variant. 

Thanks Jaidepp, but selecting routes in the Inventory tab creates the problem I want to solve. According to my understanding, you can either select "Buy" OR "Manufacture", but you can't select both. When you do, only the first rule on the list is applied. So Odoo will trigger a purchase for the product even if I have a BOM for that product.

Hi Manuel,

Since you use product variants which either have buy (variants 4,5 and 6) or manufacture (variants 1,2 and 3) you can make that specific selection while on replenishment.. Inventory > Operations >Replenishment 

On the product (inventory tab) (product alpha) you would need to select both buy and manufacture.

The above solution assumes you are on version 14 or above and have set-up the product Alpha with variants 1,2,3,4,5 and 6 
Hope this clarifies.

BR

Jaideep
Thanks for the response. I did try this but Odoo selects the first option in the order they are shown in the Inventory tab. It does not go to Replentishment so I can select the option. Maybe I have something configured incorrectly that is triggering the actions automatically, but I don't know what that could be.

I would start with revalidating the configuration

1. On the product page inventory tab buy and manufacture route is selected. Ensure no other route is selected like MTO
2. On replenishment page, remove all filters on the search bar
3. Validate if each product variant has an associated rule
4. Rule : min 0, max 0, multiple 1, trigger ‘manual’ and preferred route ‘buy’ or ‘manufacture’ depending on the variant

With the above confirmation a sales order would create a replenishment, you can trigger the route by clicking ‘order once’

2 reasons i can think off hand for replenishment not being created a. the trigger on replenishment rule is set to ‘auto’ or b) there is another route taking preference like MTO either set on product or on category which is not visible on product page.

Also if you could confirm if each variant has its own BOM or a common BOM