Source: Make Money Online with Affiliate Marketing.
Affiliate sub-IDs are a method for affiliates to track things like campaigns and member referred sales or leads.
While the basic affiliate links are enough for affiliates just getting started, I’d highly recommend adding in sub-IDs to any affiliate that begins to scale up their efforts to a big site or multiple sites.
The sub-IDs enable affiliates to see which links are performing and where, as opposed to just having overall data on performance for each advertiser.
And if an affiliate is running a site where they give back some sort of reward to members, sub-IDs enable the tracking for that, too.
Sub-IDs are just an extra bit of unique information that is added to each affiliate link.
One hassle is that sub-IDs are a little different in name and how to use them for each affiliate network.
For instance, the now defunct Google Affiliate Network called them a Member ID (MID).
The way you would add a sub-ID/Member ID with Google Affiliate Network was to simply append your affiliate link with &mid=xxxxx, where xxxxx was a unique identifier you assign to the member or website you’re tracking.
The Google Affiliate Network Member ID’s could contain up to 256 characters with all special characters in HTML encoding.
Be sure to check the help section for each affiliate network on their particular format for setting up sub-IDs.