A Sub-ID (or Sub, S1-S5, depending on the tracking platform) is a value appended to an affiliate tracking link that lets a media buyer break down performance at a much more granular level than the network's own reporting typically provides — by specific ad ID, ad set, placement, or creative variant, for example. The network passes the Sub-ID back through the postback so it shows up against the resulting conversion.
Passing Meta's own dynamic parameters (like ad ID and ad set ID) as Sub-IDs is standard practice for performance media buyers, since it lets them see exactly which specific ad drove a converting click, rather than only knowing that the overall campaign converted.
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