The "exists" option is used to make sure that the account with the incoming username must exist on this RSCD host.
If the incoming request is from ROLE:USER, account with name "USER" must exist on the RSCD host.
if the incoming request is from "NSHUSER", account with name "NSHUSER" must exist on the RSCD host.
The "exists" option is not for the mapped user, but it is for incoming user.
Hope this helps.
Thanks for pointing out the obvious that I missed. Not sure how I read that as anything but how you explained it.