In order to predict/calculate object properties in future time (within limits of calculation) one need initial position and speed at some initial moment of time.
For calculating positions and velocities, numerical method for solving differential equations is used.
There is no limitation in parameters - 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 same behavior as in real world with real objects - again, within variables, calculation and display limitations.
You can input the properties of object(s) via software interface or directly in simple TXT file.