A nested type is a type declared within the scope of another type. Nested types are useful for encapsulating private implementation details of the containing type. Used for this purpose, nested types should not be externally visible.
Do not use externally visible nested types for logical grouping or to avoid name collisions; instead, use namespaces.
Nested types include the notion of member accessibility, which is not clearly understood by all audiences.
To fix a violation of this rule, if the nested type is not meant to be externally visible, change the type's accessibility. Otherwise, remove the nested type from its parent. If the purpose of the nesting is to categorize the nested type, use a namespace to create the hierarchy instead.