Before revision:  This is because most statistical learning algorithms strongly rely on the i.i.d.~assumption on source/target data, while in practice the target data often come from a different distribution than the source data . Domain generalization (DG) aims to achieve OOD generalization by using only source data for model learning.
After revision:  This is because most statistical learning algorithms strongly rely on the i.i.d.~assumption on source/target data, while in practice domain shift between source and target is common . Domain generalization (DG) aims to achieve OOD generalization by using only source data for model learning.