A question about using idata class

Hi,
I am new to C166 and doing a project with it. I've read in some papers that variables that need to have a quick access, can be defined in this class. I want to know what are these situations? And also what happens if we don't define these variables in idata? for example does it cause to malfunction of the program on not?
And also for example is it necessary to define an index variable (that is used frequently in the program) in idata class?

Thanks so much

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