A custom tracking parameter appended to an affiliate link that allows publishers to identify which specific content piece, placement within a page, or marketing campaign generated each affiliate click and conversion — the primary tool for publisher-side affiliate attribution and content performance measurement. SubIDs (also called sub1/sub2 parameters, custom tracking parameters, or campaign IDs depending on the network) are recorded alongside conversion data in affiliate network reports, allowing publishers to analyze performance by content source. How subIDs work: the publisher adds a subID value to their affiliate link when generating it through the affiliate network's link builder (e.g., appending `&sub1=review-post` to the tracking URL); when a conversion occurs through that link, the network records the sub1 value in the conversion report; publishers can then filter or group commission reports by subID to see which content placements generated each conversion. SubID structure examples: by content piece: `sub1=product-review-2028`; by content type: `sub1=gift-guide&sub2=item-5`; by traffic source: `sub1=email-newsletter&sub2=january-send`; by placement within content: `sub1=review-post&sub2=in-text-link` vs. `sub2=sidebar-cta`. Value for content optimization: without subIDs, publishers know their total affiliate revenue but not which content drives it; with systematic subID tracking, publishers can identify their highest-converting content types, optimize link placement within content, evaluate campaign performance, and prioritize content investments that generate the most affiliate revenue. Network support: virtually all major affiliate networks support at least one subID parameter; network-specific parameter names vary (CJ uses SID, Impact uses SharedID/subid1, ShareASale uses afftrack).
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