What is the purpose of using sub ID in a link?

You can use different sub-IDs for different blog posts to determine which post is performing well, i.e., which affiliate links from a particular blog post are receiving more clicks and generating more revenue.

Similarly, you can use different sub-IDs for different deals and offers that you use in your content, allowing you to identify which of them is attracting more clicks and revenue.

Similarly, if you are cashback publisher and want to track the users, you can pass the user id in sub id in the tracking link and measure performance accordingly.

Likewise, if you are network and wants to pass click id to us, you can pass the same in sub id parameter.

Cuelinks provide upto 5 subid parameters in the tracking link to be apprended.

You can see the sub id level data in Report-> Transactions.