custom domain

From what I gather, this refers to the email address you sign up with.

If you use something like a non-gmail email address when signing up, it starts you off on the business plan with a trial (which you can instantly change to free). (Note: they’re gonna change this auto-detection thing with shared domains soon due to a security hole.)

I believe you can still use a custom domain (instead of the randomised *.ts.net provided one) with DNS lookups in your tailnet, on the personal (free) plan.