A sub ID is a custom parameter an affiliate appends to their tracking link to label traffic at a more granular level than the tracking link itself — by creative, ad placement, landing page variant, sub-affiliate, or traffic source — so that performance can be broken down and compared within a single campaign rather than only between campaigns.
Definition
A sub ID is a custom parameter an affiliate appends to their tracking link to label traffic at a more granular level than the tracking link itself — by creative, ad placement, landing page variant, sub-affiliate, or traffic source — so that performance can be broken down and compared within a single campaign rather than only between campaigns.
Most affiliate trackers and networks support several sub ID slots per link (sub1, sub2, sub3…), which the affiliate populates with their own labels at click time; the values then appear in the network's reporting dashboard, letting the affiliate see, for example, which of five ad creatives or which of three sub-affiliates actually drove the deposits.
In context
For affiliates, disciplined sub ID usage is one of the highest-leverage, lowest-cost optimisation habits available — without it, an affiliate running multiple creatives or landing pages inside one campaign can only see the campaign's blended result and has no way to identify which specific variant is actually working, which directly limits how quickly they can cut losers and scale winners. Content explaining sub IDs should walk through a concrete naming convention (e.g. sub1 for creative ID, sub2 for placement, sub3 for landing-page variant) so readers can set up a consistent system rather than an ad-hoc one that becomes unreadable at scale.
A practical point worth making explicit is that sub ID data is only as useful as the network's reporting supports — some networks pass sub ID values through to conversion-level reporting (showing which sub ID actually converted, not just which was clicked), while others only report sub ID performance in aggregate or not at all, which materially limits optimisation value; this is a genuine, checkable differentiator when comparing trackers and affiliate networks.
Content should also note the practical limit on sub ID length and character set that most platforms enforce, and the common mistake of overloading a single sub ID slot with multiple pieces of information separated by a symbol, which becomes hard to parse back out in reporting — a clean one-variable-per-slot convention scales much better.
Worked example
An affiliate running five ad creatives inside one campaign tags each with a distinct sub1 value, discovers via the network's conversion-level reporting that one creative drives 60% of deposits despite only 20% of clicks, and reallocates budget accordingly — an optimisation invisible without sub ID tracking.
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