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.