This means that: Not all words appearing between guillemets are necessarily keywords, and words appearing in guillemets do not necessarily represent stereotypes. Source Unified Modeling Language 2.5.1
In addition to identifying keywords, guillemets are also used to distinguish the usage of stereotypes. Source Unified Modeling Language 2.5.1
In cases where the metaclass of a subject is ambiguous, the keyword . corresponding to the notation for the metaclass of Classifier (see 9.2.4) shall be shown in guillemets above the name. Source Unified Modeling Language 2.5.1
Where a subject is a Classifier with a standard stereotype, the keyword for the stereotype shall be shown in guillemets above the name of the subject. Source Unified Modeling Language 2.5.1
