What is the meaning of the planned "private protected" C# access modifier?

Here are all access modifiers in Venn diagrams, from more limiting to more promiscuous:

private:
enter image description here

private protected: - added in C# 7.2
enter image description here

internal:
enter image description here

protected:
enter image description here

protected internal:
enter image description here

public:
enter image description here

According to "Professional C# 2008" by De Bill Evjen and Jay Glynn, page 1699:

private protected - "only derived types within the current assembly"

C++/CLI has a similar feature - Define and Consume Classes and Structs (C++/CLI) > Member visibility:

private protected -or- protected private - Member is protected inside the assembly but private outside the assembly.

Related