halyard [R19]: A line used for raising and lowering a sail, spar, flag, or yard on a model yacht.
main halyard [R36]: The line used on a sailing vessel to raise and lower the mainsail. On a model yacht it is often replaced with a control loop.
There are two halyards on a model yacht: the jib halyard [R25] and the main halyard [R36].
The jib halyard controls tension on the jib. See the jib halyard [R25] page for details.
The main halyard is really just a control loop [R10] that attaches the mainsail [R25] to the mast crane [R38]. See the mast head detail drawing below, which shows a wire loop with sleeve [R53]. The mainsail tension is adjusted with the downhaul [R13].
