When using the DSTT, Moonshell appears in the main menu, as a game would. Selecting Moonshell, in the menu launches the application and basically allows you to view whats on the card i.e. games/music/photos/movies etc... I'm fairly certain only 1 game or 1 app can be in use at any one time.
As I have found over the last few days whilst playing with Moonshell, there is no code for DSTT to perform a soft reset from Moonshell back into the main menu. The only way to get out of Moonshell at the moment is by powering the DS down via the switch, or via a menu option in Moonshell 1.71.