This is a unique ID for a joystick for the time it is connected to the system, and is never reused for the lifetime of the application.
Defined in SDL_joystick.h
typedef Sint32 SDL_JoystickID;
If the joystick is disconnected and reconnected, it will get a new ID.
The ID value starts at 0 and increments from there. The value -1 is an invalid ID.