The man page states:
zfs send [-eL] [-i snapshot|bookmark] filesystem|volume|snapshot
Generate a send stream, which may be of a filesystem, and may be
incremental from a bookmark. If the destination is a filesystem or
volume, the pool must be read-only, or the filesystem must not be
mounted. When the stream generated from a filesystem or volume is
received, the default snapshot name will be (--head--).
I believe the destination is always stdout and never a filesystem or volume.
However the source can be a filesystem or a volume and I think it makes sense modifications should not be possible while sending.
IMO the text would be better understood (at least by me) if "destination" is replaced with "source".