When the model has multiple objectives, this attribute is used to query or modify objective coefficients for objective
$n$. You set $n$ using the ObjNumber parameter. Note that when ObjNumber is equal to 0, `ObjN` is equivalent to Obj.

The number of objectives in the model can be queried (or modified) using the `NumObj` attribute.

Please refer to the discussion of Multiple Objectives for more information on the use of alternative objectives.