The ISelection interface indicates the method used to come up with the recommended list. The default value for the Treatment object is EXTERNAL_LEARNING so you do not have to set this value. The value is ultimately stored into Detailed Contact History for reporting purposes.
You can extend this interface beyond the existing constants if you want to store the data for analysis later. For example, you could create two different learning modules and implement them on separate server groups. You could extend the ISelection interface to include SERVER_GROUP_1 and SERVER_GROUP_2. You could then compare the results of your two learning modules.