They are "internal" environment variables used within SONAR and to expand to their corresponding per user paths.
I cooked these up to allow the installer to set up various default content location presets as used in the media browser. Since the installer does not know the current user this allows it to use abstract paths that get expanded to the right ones at runtime in SONAR. They are currently only used in browser presets.
The current list is:
%USERDOCUMENTS% // my documents
%USERMUSIC% // my music
%PROJECTFILES% // project files folder
%PROJECTTEMPLATES% // project templates folder
%TRACKTEMPLATES%" // track templates folder
%TRACKICONS% // track icons folder
%PROJECTAUDIOFILES% // project audio files folder;
%VIDEOFILES% // video files folder
%PLAYLISTFILES% // playlist files folder
%STEPSEQUENCES% // step sequences folder