restrictAffectedTypeOnServer
which is called in an onAfterAdd/onAfterUpdate server event on the Express ECO Affected Item.
The standard functionality of the Product Engineering solution in Aras Innovator would see Parts being handled by ECOs and Documents and CAD items being handled by DCOs. If this process doesn't work for your use case, you should be able to modify that Method to remove this restriction on which ItemTypes can be added as an Affected Item.
Chris