What is the Nested Structure in ABAP Dictionary
A Nested structure is a structure that refers at least to one other structure but do not refer to table type.
To create the nested structure is the same with flat structure you can use SE11 SAP Transaction code.
You can entered the name of Data Type, on this example ZSD_TEST_NESTED. and select Structure at the new screen.
The different between flat structure and a nested structure, in nested structure you have to entered at least one a flat structure in Component Type. You can see following picture.