SAP ABAP Question: Download SAP ABAP PDF

On which event we can validate the input fields in module programs?

In PAI (Write field statement on field you want to validate, if you want to validate group of fields put in chain and End chain statement.)

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What are client dependant objects in ABAP / SAP?	In selection screen, I have three fields, plant material number and material group. If I input plant how do I get the material number and material group based on plant dynamically?