Good question.
Domain – This represents the company-level account (e.g., Acme Company). You are billed based on the total active unique users (not including viewers) across all your workspaces.
Workspace – Each domain can contain multiple workspaces, each with its own users, design rules, assets, and settings (e.g., sub-brands)
Account – These are individual user profiles within a workspace (e.g., John Doe).