The default series of an element is defined within the element's assigned Value Settings.
For example, if you select Value Settings from the Model tab of the ribbon, the Value Settings window opens. If you select one of your value settings (e.g. Maximize) and then click Edit..., you will see the Default series (e.g. Actual) in the Value Settings Properties window.
To assign a default series in your formula, use the reserved word _DEFAULT. For example, the following formula will get the default series of the element with the element identifier MEA1: MEA1._DEFAULT(). For a visual illustration that demonstrates how the default series parameter works, go to How Formulas Work in Practice.