You can highlight any part of your project and then mix down only the selected area if you choose the "Use Selection" option.
1. Select any part of your project.
2. Choose "Mix Down to..." from the File Menu, then choose MP3 or another format.
3. Click to add a check next to "Use Selection."
4. Name the file.
5. Click Save.
This can come in handy if your project is mixing down with extra silence at the end, or if you want to make a loop.