Preventing a substitute from processing a workflow task

[Susan Keohan]

Hello,
 
I have a situation where it is appropriate for a user to set up a
substitute to process certain work items, but not all workitems (not
specific tasks).   The documentation on this topic refers to entries in
table T77So, using classes on the tasks, and defining substitute
profiles. But somewhere along here, I suspect I am missing the key piece
of information that will link it all together.  Has anyone used classes
to prevent substitutes from executing workitems and if so, can you
elaborate ?
 
Thanks
Sue