A General container represents a grouping of elements. Whether the elements are optional or mandatory may be inferred by the context of the container in the diagram.
A Mandatory container represents a set of elements where all of the elements described within must be present. Represents an explicit expression that the elements are mandatory.
An Optional container represents a set of elements that may, but does not necessarily, contain any of the elements described within. Represents an explicit expression that the elements are optional.