Puzzled by basic version control operation - how do you commit a change?

I'm a bit puzzled by the source control system. How do I commit a change? I see how to get a list of all the changes in a file, but how do I commit one of them? There's a "remove" command, and a "revert" command, but there's no "commit" command that I can find. (Other than the commit button in the upper left, but that commits every change in the file.)