What is Custom tracking domain?
Serving open-pixel and click-tracking links from a subdomain you own (trk.yourdomain.com) instead of the tool's shared tracking host.
When tracking is on, every link in your email is rewritten to go through a tracking host, and the open pixel loads from it. If that host is shared by thousands of senders, its reputation is shared too — and one spammer can get the host blocklisted, taking your links with it. A custom tracking domain is a CNAME on your own subdomain, so the links in your mail match your sending domain.