If you have a lot of text files on your computer, distinguishing between them can take a lot of effort. Over on How-To Geek, they remind of us the oft-forgotten feature in OS X called Summarize.
As the name suggests, Summarize will summarize a block of text. First, you'll need to enable the feature though. Head to System Preferences > Keyboard > Shortcuts, select the Services option, and find Summarize. Click the checkbox to enable it. Now, head over to you text editor of choice, select a block of text, then right-click and select Services > Summarize. You'll see a pop-up where you can move a slider back and forth to change how big the summary is. From here, you can save the summary as a new bit of text, or even better, copy and paste the summary into the Comments section of the document itself so you can easily distinguish it later on.