Answer
Ticked, it silently ignores a retry attempt to read a file that's no longer there by the time the reader gets to it (which can legitimately happen if the file was removed or changed after being listed but before being read) — useful for continuously-polled inbound directories or large files that take time to finish writing. Unticked, a read attempt against a file that no longer exists is instead treated as a failure, leaving a corresponding entry with Read Failed status in the Connect Reader Queue rather than being silently skipped.