Commerce 2.x confusion about multiple order types and order item types I have two different order types and two different order item types. Let's say they're "Default" and "Custom". On the configuration page for the Default order item type, I've specified Default as the Order type. On the configuration page for the Custom order item type, I've specified Custom as the Order type. For both order types, I use the "Inline entity form - Complex" widget for the Order items. I've set the "Allow users to add new order items" option and left the "Allow users to add existing order items" option unselected.

I expect that when I create an order of type Default, I will only be allowed to add Default order items. And when I create an order of type Custom, I will only be allowed to add Custom order items.

However, what I get instead is the option to choose from a select list whether I want to add a Default or Custom order item, regardless of the order type. So it's possible to create orders that have a mix of Default and Custom type order items. I do not understand why this is so. It seems that the "Order type" value for order item types is being ignored.

Is there a way to set up commerce order/order item types so that an order item can only be added to an order if the order type is the one specified in the order item type configuration?
As BojanZ (the maintainer of the module) has stated in his comment above, you have located a bug :) Congrats! Post it to the Commerce issue queue here:

https://www.drupal.org/project/issues/commerce?categories=All