Why does adding code from UE4 editor add this _API thing?

Development Programming & Scripting
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USTRUCT(BlueprintType)
struct PLACEBUILDINGS_API CityUpdateVO
See how it appears? Why did it add PLACEBUILDINGS_API ? PlacementBuildings being the name of my project.

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It a macro and if not mistaken it same as ENGINE_API makes your classes and function and other stuff declared in header files accessible to other modules (but hey are accessible via base classes in other modules), if you would no have that you would have linker errors if you would try to access them on other module.

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Duplicate question with staff answer: What does COREUOBJECT_API & ENGINE_API macro? - C++ - Unreal Engine Forums