A directory whose `sticky bit' is set
becomes an append-only directory, or, more accurately,
a directory in which the deletion of files is restricted.
A file in a sticky directory may only be removed or renamed
by a user if the user has write permission for the directory and
the user is the owner of the file, the owner of the directory,
or the super-user.
This feature is usefully applied to directories such as
which must be publicly writable but
should deny users the license to arbitrarily
delete or rename each others' files.
Any user may create a sticky directory.
for details about modifying file modes.