According to UML documentation (16.2.3.3 Pins):
A Pin is a kind of ObjectNode (see sub clause 15.4), so it holds object tokens that contain values of a specified Type (see sub clause 15.2 about tokens). ... A Pin is also a MultiplicityElement. The multiplicity bounds on a Pin constrain the total number of values that may be input or output by a single execution of an Action, not the number of tokens it contains (see the upperBound property inherited from ObjectNode). ... Pin multiplicity is not unique because it may hold multiple tokens with the same value.
So it appears that values and tokens are counted differently, i.e. a pin may hold, say, three tokens all referring to a a single value. And if pin have upperBound (inhireted from ObjectNode) equals to 3 and lower (inhereted from MultiplicityElement) equals to 1, it is just fine.
But then 16.2.3.4 Actions and Pins in Activities says:
Executing an Action in an Activity requires all of its InputPins to be offered all necessary tokens, as specified by their minimum multiplicity ... When the Action begins executing, all the InputPins accept tokens offered to them at once, up to the maximum multiplicity allowed on each InputPin.
So multiplicity refers to number of tokens offered!
Is there any passage in UML documentation that resolves the confusion?