preferred.mediaobject.role — Select which mediaobject to use based on
this value of an object's role attribute.
A mediaobject may contain several objects such as imageobjects.
If the parameter use.role.for.mediaobject is
non-zero, then the role attribute on
imageobjects and other objects within a
mediaobject container will be used to select which object
will be used. If one of the objects has a role value that matches the
preferred.mediaobject.role parameter, then it
has first priority for selection. If more than one has such a role
value, the first one is used.
See the use.role.for.mediaobject parameter
for the sequence of selection.