A project (*.vpj) in SlickEdit® is simply a collection of source files, along with other information that is necessary to work with these files. The data for each project is stored in an XML file that has the extension .vpj.
A project can include information like a working directory, INCLUDES or CLASSPATHS, compile and build command lines, and more.
If you are building/compiling something that only contains one source file and do not want to create a new project for a single file, then you can define an extension-based project tool to do the build/compile instead. Extension-based project tools are based on the extension or file type of the current file; however, the working directory is ignored for extension based projects.