Sub-IDs let affiliates add custom tracking to their links, enabling detailed performance analysis of their promotional efforts. Sub-ID uses: - Track different pages/content - A/B test creatives - Segment by traffic source - Track by product category - Monitor individual campaigns Sub-ID example: yoursite.com/?ref=aff123&sub1=blog_review&sub2=header_link Best practices for brands: - Support multiple sub-IDs - Pass through to reporting - Enable affiliate analysis - Document usage Benefits: - Affiliates optimize better - Better partnership insights - Identify top-performing content - Inform collaboration strategy