Hi Chris
From version 2.7.0, the License code is used for different purpose and stored in different table (#__dms_order_documents table). The purpose of license code is that it allows you to manage license code for each document (you can got to Documents Seller -> Licenses to create license code). License will act as Serial number (or CD Key....) which customers need to enter to use your product (software in this case). I think this feature is only needed if you sell software. If you just sell normal files download, then no need to use this feature.
When customer purchase a document, the system will find from one of available license code for that document and will send to customers in confirmation email.
Hope my explanation is clear to you.
Tuan