The global variables in Reframework are accessible through out the framework.
For eg. You have seen the key value pair dictionary variable (name Config) in standard Reframework template which UiPath does provide. This variable is accessible in all states init, get transaction, process and end process state.
Work flow analyzer is tool that can capture the work flow errors and make our automation more robust and maintain all the best practices. Work flow analyzer comes with inbuilt rules that will help us to automate better.
Answer for your question. Yes we can create our custom rules in work flow analyzer if your process is demanding. For that you have to install uipath.Activities.api package and need to follow some steps. Please refer the below documentation it will help you to achieve.
Global exception handler is exception handling mechanism in studiox. It is pre filled with the workflows it will help business users to handle thier exception in their automation. If you get any exception in studiox by default it will go to the global exception handler and it will display one popup and it will ask us to retry or stop or continue. If it is temporary issue we can retry our process.
If you want you can modify the work flows in the global exception handler and we can add our work flows to handle exception in different way. Please refer the below documentation for more guidance. Thanks.
Global variables is generic term which we can create and use anywhere in the our process.
In Uipath mostly we keep global variables in two places. As mentioned by @shreyash_shirbhate we can keep global variables in config file as dictionary variable like file path and application url and we can access anywhere in our process and second place would be assests in orchestrator. Mostly assets are our app credentials. Once we create assests we can access these in our entire process by calling with get asset activity. I hope you got some info on this. Thanks.