Question
Please refer to the following diagram which is shown in the UML 2.5.1 specification at section 11.5.2 on page 241 of the PDF file:
What is the difference here? navigableOwnedEnd subsets ownedEnd, and due to the dot notation, both are navigable by default. The descriptions on page 261 are basically the same.
Solution
No, they are not the same. An Property can be owned by the Association or by the Class. If it is owned by the Class it is automatically navigable. If it is owned by the Association it is only navigable if it is a also a navigableOwnedEnd.
That means the concrete syntax of the navigability arrow has two mappings to the abstract syntax.
Solution
These are two different concepts, as UML specs state page 202:
Aggregation type, navigability, and end ownership are separate concepts, each with their own explicit notation.
End ownership is a structural characteristic whereas navigability is a runtime promise. See this OMG article for an in-depth analysis.
While navigability is assumed with the dot notation, UML specs explicitly allow for a combination of navigability and dot notation, page 202:
The dot may be used in combination with the other graphic line-path notations for Properties of Associations and Association ends. These include aggregation type and navigability.