How Batch Numbers, Inventories, and BOMs work when using Multi-Inventory & Product Levels Together.
After finding unanswered questions and little documentation related to this topic, I figured it out after a few hours of tinkering with test cases on my website. This is what I discovered.
If you are working with BOMS, nested or otherwise, everything related to inventories and batch numbers happens in the background. It is not easily apparent (perhaps not possible\recommended) how to choose inventories and you cannot see the associated the batch numbers of any BOM part or material during the order fulfilment process. Everything is generated automatically when an order status progresses further than "Payment Pending". However, it may be possible to make edits afterwards.
Here's an example:
An order is placed for a manufactured products, Product A, The product is associated with a BOM containing 10 materials. Product A has 2 inventories, Main Inventory, and an inventory I called On Hand which typically includes products that I made in advance. I could have associated each of those inventories with a Batch number on the product page, but I'm mainly interested in the Batch numbers of my materials. I have not selected the option to assign product inventories automatically in ATUM, Settings, Multi-Inventory. As a result, when I open the order I must click on the edit symbol next to Product A and then click on the Add Inventory button. For this case, the Add Inventory button will provide two options, Main Inventory and On Hand. For the purposes of this example I will select Main Inventory. This displays the BOM tree below the product listing, the individual materials and the amount required of each. However, it is not possible to choose inventories for the 10 materials and there is no indication of which inventories will be used. Once you change the order status to processing or complete, the inventory names will be automatically generated next to each material, based on the inventory iteration and priority settings you have assigned.
In the image captured from a test order I created you can see a nested BOM tree with part names and the automatically generated inventory name.
If you change the order back to pending payment you can edit the automatic inventory selections:
Unfortunately it looks like the process of switching orders from Complete or perhaps processing back to payment pending causes some issues when re-assigning inventory back to the the original product. It's likely this workaround is not recommend.
I would recommend setting your inventory priority correctly and not making changes.
So what about Batch tracking?
Despite being unable to see a trail of batch numbers in the logs, I confirmed that you can search for the batch number associated with any material or part used in a BOM and it will bring up the associated orders. I imagine all the other batch number searches work equally well. As for what happens with Batch number tracking if you try my inventory editing trick above, I have no idea.
I imagine a manufacturing order feature might remedy some of these issues. I would welcome feedback on the input I've provided above and would be interested in knowing the easiest way to see a list batch numbers associated with an order. Good Luck!