SAP has provided standard set of fields in the customer master to be used for various purposes. Some fields are per-defined for certain purposes with an attached functionality behind. In order to provide maximum flexibility for SAP users, there are some fields which are not pre-defined, but can be used for custom purposes such as; Customer Group 1 to 5. But those fields are depending on the sales area of the customer. If you want to define a value which is independent of the sales area, you can define your own classification for customers. This classification can be attached to customer master with different characteristic values. Since this is not sales area depended, you can define it as a global value for the customer master.
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