A sub-ID is a custom value an affiliate appends to their tracking link to label where a click came from: a traffic source, campaign, ad set, keyword, or creative. The network stores the sub-ID with the click and returns it on any resulting conversion, so the affiliate can see which segments of their traffic actually convert.
How it works
Most affiliate platforms expose one or more sub-ID parameters, often named sub1, sub2, and so on, or subid. The affiliate fills them with their own values, either manually or dynamically using a traffic source's macros. When a conversion is attributed to that click, the sub-ID values appear in reports, exports, and postbacks.
Sub-IDs are not the same as a click ID. The click ID is generated by the tracker and unique per click; sub-IDs are set by the affiliate and shared by every click in the same segment.
Example
An affiliate promoting one offer in two newsletters and a paid search campaign might use:
https://track.example.com/c/88?sub1=newsletter_a&sub2=banner_top
https://track.example.com/c/88?sub1=newsletter_b&sub2=text_link
https://track.example.com/c/88?sub1=paid_search&sub2={keyword}
In the third link, {keyword} is a placeholder the ad platform replaces with the actual search keyword. Example: if reports show 40 conversions on newsletter_a and 3 on newsletter_b from similar click volumes, the affiliate knows where to move budget.
Why it matters
For affiliates, sub-IDs are the cheapest optimization tool available: they turn one tracking link into many measurable segments. For brands, sub-ID data supports quality control. If fraud or low-quality leads cluster on one sub-ID, you can ask the partner to cut that source instead of pausing the whole account. MassPartner affiliates get personal tracking links with sub-ID support; see how it works for affiliates.
Related: tracking link, tracking macro, postback URL.