orderBy: Clarify meaning of "A special case of such an expression is a property path terminating on a primitive property"

[colgreen]

What does this sentence mean (in the section about orderBy)?

The value of the orderby parameter is a comma-separated list of expressions used to sort the items. A special case of such an expression is a property path terminating on a primitive property.

Surely all orderBy strings refer to a specific property. What is meant by 'primitive' property, and why is that relevant here?

Thanks.