Requires integration code to be provided for each entity type that can be referenced. Supports the commerce_product (including Commerce AutoSKU integration) and commerce_line_item, node, taxonomy_term entity types out of the box.
I have two custom types, products and purchases. products is the master, purchases are individual line items.
I have tried following the instructions but no widget type "Inline entity form" is ever available and certainly no "purchases" editable via the "products" form...
There is some integration code required to make use but the docs also state it works out of the box for "nodes" as I have boldified above...are custom types not "nodes"???