It is possible to designate objects to be part of one or many trace sets. The trace_set shader marks specific rays with an inclusive or exclusive trace set. Both geometry and rays can be designated as trace sets:
The way those two interact makes it possible to control visibility for specific rays:
The shader that gets called after setting the label.
The string label defining the set of objects to be traced or avoided. Objects are labeled using the Arnold Parameters Tag property.
If on, the tracing works in inclusive mode, else in exclusive, as described above.