value for selecting which of several objects within a mediaobject to use.
The order of selection when then parameter is non-zero is:
If the stylesheet parameter
preferred.mediaobject.role has a value, then the object whose role equals that value is selected.
Else if an object's role attribute has a value of
html for HTML processing or
fo for FO output, then the first
of such objects is selected.
Else the first suitable object is selected.
If the value of
is zero, then role attributes are not considered
and the first suitable object
with or without a role value is used.