A new document will be created at launch. You can make new documents using "New" from the "File" menu.
You can save and open Soulver files as you would in a text editor. Soulver files remember your insertion point, aling with the formattion options you have setup for your lines.
Tips
If you find the save warning when closing documents annoying, you can disable it in the preferences.
Go to the Soulver > Preferences and enable "Supress save warning for unsaved documents".
Soulver can open your last saved document at launch, instead of making a new document each time you open.
Go to the Soulver > Preferences > General and choose "Open the last saved document" from the "On launch" popup menu.
You can use Quicklook in Finder to preview Soulver files.
Soulver files are based on XML and can be opened with a plain text editor. If you are a developer, you can easily read Soulver files.