Hello Shahaji,
Global code is something that is available to entire object and that will loaded in memory when you call any of action under the object.
For more understanding, open any action of MS Excel VBO, there you will find method called GetInstance(handle) method.
Now the question is where the GetInstance Method has been declared?
The answer is inside the global code of intialize page property.
NOTE: Please do not change any method. Any change in method or action or object may stop working the particular object. While exiting, please discard the changes.
Thank you.
Regards,
Vijay Rudrappa Dodamani