Element's that contain elements are typically modeled:

• sequences of elements (e.g. A followed by B followed by C)

• choices of elements (e.g. A, B, or C)

• occurrences (e.g. one or more, between 5 and 10, optional)

• all of these elements in any order

• some of these elements in any order

Models allow combinations of the above. (but there's restrictions)