Glossary
SubID
A SubID is an additional parameter you can attach to an affiliate link to label where the click came from. It's meant to enable link- or placement-level tracking.
Why it matters for your earnings
In theory SubIDs give per-link data, but most programs — Amazon included — don't report SubID-level earnings to ordinary affiliates, so for most people the tracking ID, not the SubID, is the real reporting unit.
Go deeper: Guide: Amazon Tracking IDs