Sub IDs are optional variables you can add on the end of your affiliate links.
They can be used for example to track different types of traffic or for example if you are running an incentive or rewards site you could use them to track which members took part in which offers. If you are promoting the offers via PPC you might use a Sub ID to put that the traffic is coming from 7Search or from Google.
Sub IDs can help you to track more information for each conversion. Useful for insights and also optimisation of traffic.