Definition
Sub-ID tracking is a system that allows affiliates to add custom parameters, typically called sub1 through sub5, to their tracking links. These parameters pass through the entire click-to-conversion flow and show up in reports, letting both the affiliate and the program manager see exactly which traffic source, campaign, or creative produced each conversion.
Sub-IDs turn aggregate performance data into actionable insights. Without them, you know an affiliate generated 20 sales. With them, you know exactly where those sales came from and can optimize accordingly.
How sub-ID tracking works
An affiliate promoting your product across multiple channels structures their links with sub-ID parameters. For example: yoursite.com/?ref=partner123&sub1=youtube&sub2=review_video&sub3=description_link. The sub1 identifies the channel, sub2 names the specific content, and sub3 marks the link placement.
When a visitor clicks that link, your tracking system captures all the sub-ID values alongside the standard click data. The sub-IDs are stored with the click record and persist through the entire conversion flow. When a conversion happens, all sub-IDs are recorded alongside the sale data.
The affiliate can then filter their reports to see that YouTube review videos convert at 4% while newsletter links convert at 1.5%. They double down on what works. On your side, you can see which types of content from each affiliate produce the best results, helping you decide where to focus co-marketing efforts.
Most tracking platforms support five sub-ID fields, but even using just one or two consistently makes a significant difference in optimization capability.
Common sub-ID naming conventions
Successful affiliates develop consistent naming conventions that make their data immediately useful.
sub1 — Traffic source. The broadest category: youtube, instagram, email, blog, tiktok, facebook. This tells you which channel the traffic came from.
sub2 — Campaign or content. The specific piece of content: review_video, comparison_post, weekly_newsletter, holiday_promo. This identifies what the visitor engaged with.
sub3 — Placement. Where in the content the link appeared: description_link, bio_link, sidebar, hero_banner, email_cta. This shows which positions drive the most clicks.
sub4 — Creative variant. If the affiliate is testing different approaches: headline_a, headline_b, red_cta, blue_cta. This enables A/B testing at the affiliate level.
sub5 — Custom data. Any additional context: audience segment, geographic targeting, promotion date. This is a catch-all for information that does not fit the other fields.
The specific naming convention matters less than consistency. When every link follows the same structure, the data becomes immediately analyzable. When sub-IDs are used inconsistently or left blank, the tracking data loses most of its value.
Why sub-IDs matter
For affiliates
Sub-IDs are how professional affiliates make data-driven decisions. Without sub-ID data, an affiliate knows they sent 500 clicks and generated 20 sales. With sub-IDs, they know those 20 sales came from their Instagram stories (12), their blog sidebar (5), and their email newsletter (3).
That granularity changes everything. The affiliate knows Instagram stories convert 3x better than email for your product, so they create more story content and reduce email sends. Their overall EPC improves, they earn more, and you get better-quality traffic.
For program managers
Sub-ID data reveals which promotional strategies actually work across your affiliate base. If you notice that video reviews consistently outperform banner ads across multiple affiliates, you can create better video assets and share that insight with your partners. You move from guessing to optimizing.
Sub-IDs are also essential for fraud detection. Requiring partners to pass traffic source data makes it much harder to hide fraudulent activity. A partner claiming all their traffic comes from a single unknown source is a red flag. One passing granular sub-IDs with legitimate traffic sources is demonstrating transparency.
For reporting
Sub-ID data enriches every report in your program. You can break down conversions by traffic source, see which creatives produce the best return, identify seasonal patterns in different channels, and spot emerging opportunities. This data also supports conversations with affiliates about optimization, giving you specific, evidence-based recommendations instead of generic advice.
Frequently asked questions
How many sub-IDs should I require affiliates to use?
Do not require all five. Start by recommending sub1 (traffic source) and sub2 (campaign or content). These two fields cover the most useful breakdowns. Make sub-ID usage easy by providing link templates and showing affiliates how the data appears in their dashboard. Experienced affiliates will naturally use more fields as they see the value.
Can I set default sub-ID values for affiliates?
Some platforms let you configure default sub-ID values at the offer or partner level. This ensures basic categorization even when affiliates do not manually add parameters. However, defaults work best as a fallback. Affiliate-provided sub-IDs are more valuable because they reflect the affiliate's actual promotional strategy.
Do sub-IDs affect tracking accuracy?
No. Sub-IDs are metadata attached to the click. They do not change how attribution works, how cookies are set, or how conversions are matched. They add context to existing tracking data without altering it. The only way sub-IDs could affect accuracy is if they are truncated by URL length limits, which is rare with modern platforms.
What happens if an affiliate does not use sub-IDs?
The click and conversion are still tracked normally. You simply lose the granular context about where the traffic originated. The conversion is attributed to the affiliate but without the source, campaign, and placement data that would make the report actionable. Encouraging sub-ID usage is about data quality, not tracking functionality.
Trcker tip
Trcker passes sub-ID values through the entire conversion flow and displays them in your reporting dashboard, so both you and your affiliates can drill into performance by campaign, source, and placement. Sub-ID data also feeds into the fraud detection engine, helping flag suspicious traffic patterns that would be invisible without source-level granularity.