A User Interaction Error occurs when the User (or an Indirect Command File)
supplies an illegal response (e.g. illegal syntax for an Integer value, illegal
Verb etc.). Such errors always cause an error message to be displayed. If
such an error occurs during the execution of an Indirect Command File, the
error message will indicate the type of error (illegal syntax, unknown response
etc.), the filename and line number in which the error was detected, and the
path by which that file was accessed (i.e. the sequence of nested Command
Files).
The default response following such an error is for all Indirect Command
Files to be closed and further input to be taken from the keyboard (or batch
input stream in batch mode). Furthermore the remainder of the Input Buffer (at
the direct input level) will be flushed such that any remaining information
that was typed-ahead into the buffer but has not yet been decoded will be lost.
This automatic unwinding and flushing can be disabled if the Application
Programmer so desires (via Subprogram UI_FLUSH_MODE).