It is defined using an IDCAMS command DEFINE GENERATIONDATAGROUP or DEF GDG for short. Not many parameters just a maxgeneration as I recall.

If you create a GDG called MY.GDG this first instance will be cataloged as MY.GDG.G00001V00 and onwards. Each time you reference one you can do so by absolute reference as above or relative. MY.GDG(0) which is the most recent generation. MY.GDG(-1) is the next oldest and so on.

To create a new GDG generation you code in your JCL ..DSN MY.GDG(+1) DISP (NEW CATLG) etc as normal. Whatever you created as a max generations the last one gets dropped off (uncataloged and deleted) assuming you have that many generations.

They are useful for backups and the like. If you run a particular batch job on a daily basis then its often the case that you have a 5 generation (or 7?!) GDG to capture your output or parameters etc. You then have a week to print them or reference them.

Model DCB is used because to avoid writting the characterstics of the generations repeatedly. The advantage is that we assign the characterstic of a dataset to all the generations by specifying the model DCB.There is a less chance of getting error.

Note: All the generations of a GDG should have the same characterstics.

GDG nothing but Generation Data Group means data sets has been generated with version number.

It will be referred as a dataset with version number then only we knows and identifying the dataset when it was created and easily identifying the data is stored in the dataset with help of version number.

BY using the IDCAMS utility we can define the GDG. Before that we can create the GDG base.