What are underscored subview names for?

[trusktr]

I see a check in the Famous AutoLayoutController for

if ((key.indexOf('_') !== 0) && (subView.type !== 'stack')) {

(here)

Why does it check for the underscore? I don't see it in the docs. Maybe the data structure should have a more meaningful property like type for that purpose, instead of relying on a string prefix?