GroupSourceInterface
Magento\Customer\Model\Customer\Source\GroupSourceInterface
What it does
This interface defines a contract for classes that provide customer group options as a data source.
Developers implement this interface when creating classes that need to supply customer group lists for dropdowns, filters, or other UI components in Magento 2.
It extends OptionSourceInterface to ensure implementing classes return customer groups in a standardized array format suitable for form elements.