Truecauses the specified frame to be a horizontal phantom frame. The frame will not appear but its vertical components (frame height, inner top and inner bottom spacing) will be used to calculate positions and dimensions (horizontal components are treated as 0).
This is extremely useful for a correct labeling of objects. You would e.g. label a string gadget by using a text object with a phantom string frame. Thus, the label text will be always on the same vertical position as the string gadget text, no matter what spacing values the user configured.