ORA-DM-ATOMIC-SINGULAR

*Data Model, Section 5. Atomic values*
The bullets in the middle of this section would be better expressed in 
the singular,
eg, "The value of a node whose type is an XML Schema primitive type ... 
is represented as an atomic value of that type".
In general, when you have implicit universal quantification, it is a 
better practice to use the singular because it avoids ambiguity.