If the input dataset is coming from a terminal (ie. a tty), WrenGROGGS will invoke a user interface (assuming that there's a user on the other end). If the input dataset is coming from a file (eg. because you have specified the /stdin switch or the from option), no user interface will be presented : data will be read and dealt with as per the command line or your preferences, and an error will be generated if it is incomplete.
If the output dataset is going to a terminal, WrenGROGGS will use a pagewaiter to display the output (unless you've set /display). If the output dataset is going to a file (eg. because you have redirected stdout or because you have specified the to option), no pagewaiter is used.
Output to the ver dataset is always dumped directly to the destination, be it file or terminal, with no processing applied. The ver dataset defaults to stderr, unless you change it with the ver option.