An Atlassian credential is a developer space identity — the trio of developer id (from your Marketplace vendor URL), the Atlassian email that owns it, and an API token minted at id.atlassian.com. Products sync as one of these: the credential is what the sync presents to Atlassian's Marketplace reporting API, so it decides which apps' licences and transactions are visible.
The API token is write-only: stored encrypted, never echoed back — on an update, a blank token field means "keep the stored one". Deleting a credential is refused while any product still syncs as it; delete or re-point those products first.
One special case: a credential may carry a mock base URL. Products on it then sync from a mock Marketplace (fixture data through the identical client, auth, pagination and retry path) instead of Atlassian — that is how the demo runs offline.