Ignore this subtree for the normal rendering process and draw it in transparent gray (all transformations are still applied to the nodes in this tree). Because the marked subtree is completely ignored, it might have unexpected effects in case it's used, for example, with the first object in a difference(). In that case this object is rendered in transparent gray, but it is not used as the base for the difference()!
And there is $preview
$preview is true, when in OpenCSG preview (F5). $preview is false, when in render (F6). This can, for example, be used to reduce detail during preview to save time, without losing detail in the final rendered result: