Thanks for writing in! A global block is an independent block or section where you can use it anywhere in your site. It is different from the header or footer. You can create a global block and use it inside the custom header or footer. Regretfully you cannot save a top or middle bars as a global block. You will have to replicate your header to have the same set up and edit each of the headers when you want to change something in the top or middle bar.
Please check this articles:
Hope this explains it briefly.