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If a GDG exists in 3 generations, how would you reference the 1st generation in the JCL?

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Generation(0) refers to the current GDG generation, Generation(-1) refers to the GDG generation just prior to the current generation, and Generation(+1) refers to the GDG generation following the current generation.

When a GDG exists in 3 generations, then the 3rd generation is the current generation, and therefore, we refer to it using generation(0) in JCL.

The generation (2nd generation) right before the current generation (3rd generation) is referenced with generation(-1) in JCL.

Lastly, the generation (1st generation) just before the 2nd generation is referenced using generation(-2) in JCL.

Conclusion

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