UML241 — Issue in UML 2 Interaction package

In the UML 2 Interaction package specification (UML
2 spec, dated 2003/04/10): p. 381, it is written that
"InteractionFragment is an abstract notion of the most general
interaction unit. An interaction fragment is a piece of an interaction.
Each interaction fragment is conceptually like an interaction by itself".

InteractionFragment is described as an abstract notion and Interaction
is defined as a specialization of InteractionFragment. Don't we define
normally in the other way around: the InteractionFragment is the
abstract notion and is called Interaction and the interaction is called
InteractionFragment? We can have the definition for Interaction
(previously called InteractionFragment): "Interaction is an abstract
notion of the most general interaction unit. An Interaction is composed
of at least an InteractionFragment".

Usual semantics for fragments refers to something within something else
but here, the something else is undefined.