If you wish to make money on the eBay Partner Network by promoting eBay links and advertisements on your website, social media platforms, blogs, and other sources, you need to create a campaign in your EPN account. Each campaign has an alphanumeric name and ID.
Campaign ID is a 10-digit number that identifies your campaign. Campaign ID is a required field for all EPN links. With a help of your campaign ID, EPN tracks the visitors that come to eBay by clicking on one of your links and also their activities on eBay, such as making bids and purchases.
As soon as you sign up for EPN you already have one default campaign. But you can create your own campaigns.
Learn how to create a campaign in your EPN account
Custom ID - an alphanumeric identifier of up to 256 characters you create to identify your eBay Partner Network traffic. You can use Custom IDs to track your traffic within a specific campaign or across multiple campaigns. For example, you can see a Campaign ID to identify traffic from a particular website, but use a Custom ID to track traffic that comes specifically from the homepage.
You don't have to create a custom ID in your EPN account, you can specify the ID in eBay Feeds generator to add it to your links and it will be automatically tracked in your EPN statistics.
Learn how to sign up for eBay Partner Network