f96c6a36_EMF_v1_0_Users__Guide__Java
[Property] nsPrefix [UML] prefix property is specified on the eCore page of the specification dialog for the UML Package. [XMI] nsPrefix attribute on the ecore:Package element in the ecore document. [Java] 
The initial value of the eNSPrefix field is a member of the interface that corresponds to the package itself.
EMF v1.0 Users' Guide