Bottom of the jib, Jerome, not the top.
It's for the jib downhaul - in the old jibs, the luff wire was fixed to the cloth at both ends, making an adjustment of the jib luff impossible. On new-ish jibs, the wire is only fixed at the head (by a metal strap) and the luff is tensioned by running a small line through the sail grommet and to the thimble at the end of the wire.
The newest jibs have the metal strap at the tack and the line at the head - the line doesn't wear on the forestay adjuster that way.
