It's software for visualization of gravitational force effects between (two) objects.
In order to predict/calculate object properties in future time one need initial position and speed at some initial moment of time (of course, within limits of calculation).
For calculating positions and velocities, for solving differential equations a numerical method is used.
There is no limitation in parameters - the object will behave just like they behave in the real world. The only thing which is not ‘real’ is an option to increase the object-sphere beyond physical meaning (unrealistic small density). So, be careful with initial data - you will see the same behavior as in the real world with real objects - of course, within variables, calculation and display limitations.
Data is calculated and displayed in SI units (second, meter, kg, meter/second etc.). For an example, you can check if your data for sun and earth are correct by watching the amount of time elapsed for one earth revolution around the Sun.