Item_Group_ID is the field mapped in your Master Fields at the shop level.
This field is locked by default. If you need to change it anyway, you need to click on the lockpad icon first.
This special type of ID is currently required and used in:
The Item_Group_ID values must be unique for the parent products and common for all of its variants.
Any change here may influence other functionalities across the app. Such as the Exclude Associated Variants & Merging Product Variants (Facebook).