UML241 — Issue in UML 2 Lifeline class

p. 386, it is written: "To depict method activations we apply a thin grey or white rectangle that covers the Lifeline line". Figure 8-144 p. 372 presents several sizes of grey rectangles. Particularly, does it mean that method activation is performed all along the Lifeline ob1:C1? A temporal vertical axis will be added to allow several sizes of rectangles and duration of method activation.