What is "in this text domain" for Ultimate Branding?

I am trying to change text for a single blog and not sitewide in our network. Specifically the BUY Now button for Marketpress. We want it to say Register - BUT only for the one site. Others may want the Buy Now text.

I thought by adding a rule and using "in this text domain" I would be able to specify the blog by simply adding the pattern such as /test/.

There is no mention of this feature in the example and usage: