UML visibility modifier ambiguity in nested class structures

The Unified Modeling Language (UML) specification defines four visibility levels—public, private, protected, and package—yet it does not mandate a strict default state for new elements when visibility is not explicitly defined. This leads to inconsistency across different modeling tools, which often default to public, potentially resulting in accidental exposure during code generation.

A specific ambiguity arises regarding the interpretation of the package (~) modifier when applied to nested classes. It remains unclear whether this modifier should grant access to the entire package or be restricted strictly to the immediate containing namespace. Furthermore, the behavior of the protected modifier varies significantly between target programming languages, complicating the mapping of model elements to source code.

How should modeling engines resolve the package visibility of nested elements to ensure cross-platform consistency, and what is the recommended approach for defining default visibility in the UML model to prevent over-exposure in the generated API?