The default project is the active project at startup (the project whose name appears in the Softimage title bar).
When you first install Softimage, the default project is XSI_SAMPLES.
After that, as you open scenes and exit and restart Softimage, the default project becomes the project that contained the scene you last opened. This is saved in your Default.xsipref file like this:
data_management.last_sequence_dir = C:\Users\SOLIDANGLE\Projects\My Project\Scenes
But you can explicitly set a default project with the Project Manager, then that is saved in your Default.xsipref like this:
xsiprivate_unclassified.DS_SZ_LOAD_DEFAULT_PROJECT = #STRI#C:\Users\SOLIDANGLE\Projects\Support