Are the elements of a substitution group mutually exclusive?  Or can more than one element of the group be used together?

Members of a substitution group are not mutually exclusive.  However, cardinality constraints on the head element apply.  So if the head element has maxOccurs=”1” (or maxOccurs is not specified, as the default value is 1), only one (but any one) of the substitution group members may be used.  If the head element has maxOccurs= ”unlimited,” than any number of substitution group members may be used in any combination and in any order.  So, for example, if elements A, B, and C are members of the substitution group, you could have such combinations as AA, AAB, ABC, CBBA, and so on (although only certain combinations likely make business sense).