This is the section where parameters are defined. Only  name and initialValue are required. Here is where you could define parameters to store the position of the second atom.

You use get('parameter-name') and set('parameter-name', value) to work with them. Here is an example of an interactive that uses a lot of parameters: http://lab.concord.org/interactives.html#interactives/inquiry-space/1-pendulum.json