passing a cell mark to UDF
 

I would like to combine two cell marks before doing a thread cell separation in Fluent. My obstacle is that I have to separate the cells by shear stress and this variable is not defined in UDF. In Adapt\iso-value i can create a mark by shear stress but I can't find the way of passing this mark to an UDF and storing it in a UDS. Could anyone help me? Thanks in advanve for any suggestion