Hi @Kumar_27 ,
Go through this document on 'Tuesday tech bytes' series, explained in detailed about task variable, event variable, instant variable, calling a variable inside a delivery and much more.
https://experienceleaguecommunities.adobe.com/t5/adobe-campaign-classic/tuesday-tech-bytes-adobe-cam...
Example, In the below Workflow there are 2 independent flows.
Consider an Event variable is created in JS-1 on 1st flow. In the same flow, it is possible to read the value of JS-1's event variable in JS-2.
But when it is tried to call the 1st flow's event variable in 2nd flow (say, JS-3), it will print 'undefined'.

So, If the requirement is to create a variable in JS-1, which can also be used in JS-3 (another flow of same workflow), then create it as instance variable.