Each preprocessor directive applies only to its own compilation unit, not the entire project. So, a #define directive in main.h will not be "visible" in main.tbs unless the latter includes the main.h.

The only exception to the above are platform defines. These are globally visible throughout your entire project. Platform defines determine options such as the presence or absence of a display, display type, etc. These options are selected through the Customize Platform dialog, accessible through the Project Settings dialog.