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I am working with unity for an iOS game. For unlocking achievement I need to access a "Achievement.mm" file from my c# code:

    [DllImport("__Internal")]
    private static extern void
       GKAchievement(string achievementID, float progress, bool value);
I got this code from a forum. But, what does the "__Internal" means ?
                It is specific to Mono, tells it to look for the function in your own program instead of another DLL.  I think also required to get your app certified by Apple.  Explained pretty well in the docs.
– Hans Passant
                Jul 26, 2016 at 11:56
Plugins link their code to C# unity using [DllImport()].
__internal is a keyword used for static linking needed by specific platforms, such as XBox and iOS.
Other platforms usually use dynamic linking, hence this is not needed.
Please for more info refer to: https://docs.unity3d.com/Manual/NativePlugins.html
Just the minor tip. 
If the library being loaded is __Internal, then the main program is searched for method symbols. This happens to be the case on iOS platform.
The plugins on iOS cannot load dynamically. Only statically as @RaxelZ explained.
It is different for OSX, Windows, Android ... those platforms may load plugins dynamically.  For iOS, only statically.
        
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