Steps to reproduce:
Result: there is a option to "Map It" at the end as well as the following text:
Already own a domain? Map this domain to use it as your site's address.
Feedback from a #dogfooding session:
What does “Map it” mean? Does that just mean “Use it”?

Seen at https://wordpress.com/start/domains using Firefox 58.0.2 on macOS 10.13.3.
/hat tip 🤠 @beaulebens for reporting this issue at p4k3M4-2uX-p2. #dogfooding #painpoint
I wonder if expanding just a bit in the first instance would make the button CTA clearer?
e.g.:
Already own a domain?
You can map an existing domain to use it as your site's address.
@designsimply we're changing that for good, as it's going to be replaced with Already Own A Domain, with transfer-in as the main action.
I'm closing this since the recent launch of the domain transfers in during NUX changes the prompt from "map it" to "use a domain I own". This is what I see now:
