Who can access it if a method is marked as protected internal?

If the method is simply marked as protected it is only visible to the inheriting types. However, if it is marked as protected internal, it is visible to classes within the same assembly and classes derived from the declaring class. This means that all members in a specific class will be able to visualize the method.

Different settings provides users with the option of modulating which people can access the method. If you want to privatize the method, you can easily do so by changing the marking of the method. Different options will make it visible to only the select few personnel that you allowed.