Example: State and <State Control Link>
Briefly, any <Control Link>, including a <State Control Link>, controls some value in a target object - in this example, a <State Related Geometry>'s active_state value.
The control itself is specified by some <Expression> provided by the <Control Link>.
In this example, the <Expression> might be a <Variable> representing the precipitation rate, for which values could be supplied dynamically by the end user, if desired.