Use of "#define FOO" with no value assigned – other than as include guard?

  c++, include-guards, preprocessor-directive

I’m trying to understand Steinberg’s VST SDK.

What’s the use of this:

#define PLUGIN_API 

without assiging any value in a header file and then PLUGIN_API occurring in many member functions’ declarations/definitions for example like this:

Steinberg::tresult PLUGIN_API initialize (Steinberg::FUnknown* context) SMTG_OVERRIDE;
Steinberg::tresult PLUGIN_API terminate () SMTG_OVERRIDE;

Can someone explain this? The only use of #define without assigning a value that I know so far is using it as include guard.

Thank you in advance!

Source: Windows Questions C++