BAPI_CHARACT_CHANGE usage

Hi,

I have a requirement to update characteristic values in SAP using BAPI_CHARACT_CHANGE.

I am able to update data using this BAPI but not getting updated for appropriate CLASS. I have same characteristic(Ex: PRODUCT_GROUP_CODE) for  couple of classes and not able to point to correct class to update the values and if executes it is getting updated for different classes.

Could anyone guide me how to point to characteristic of exact class? Here I have to update PRODUCT_GROUP_CODE for exactly to class FINISHED_GOODS, but as per my approach of code, sometimes it updates for POS class .