They can be the same domain, but we generally recommend using a subdomain for each so they're easier to manage separately. Your sending domain is what appears in the From address (e.g. mail.yourcompany.com) and needs SPF/DKIM/DMARC set up. Your tracking domain is used for click/open tracking links and unsubscribe links, and just needs a CNAME pointing at our tracking servers. Using a subdomain like links.yourcompany.com for tracking (separate from your sending subdomain) is a good default — it keeps your root domain's reputation isolated from tracking-link behavior.