This tag file indicates to the actor that the directory valid as a run
directory for the actor.  It helps prevent accidental destruction of
data by entering a valid, but inappropriate directory name as the
run directory name. Every run directory or backup directory for this
actor must have this tag file.  A directory which is a valid run
directory for more than one actor type would have tag files for
all relevant actors.  The contents of this file are not important,
it simply needs to exist in the directory.