Constant UNPLACED_PORT_LABELS_KEY 

pub const UNPLACED_PORT_LABELS_KEY: &str = "\u{1}unplacedportlbl";

Edge attribute set by a layout whose head or tail label it could not place (ED_head_label(e)->set false: h, tail: t): head_lp/tail_lp are still printed, but renderers do not draw the label.