They lift more weight than the buntlines, and also have to pull against the sheets - although these will have been released there is still a certain amount of friction produced by the blocks and fairleads that they run through.
The clewlines are coloured green in the diagram, and run along the underside of the yard from the outboard ends to the mast, and then down to the deck.
A typical arrangement for the buntlines has them running through deadeyes on the yard, upwards to a block fixed to the shrouds a little higher up the mast, and then downwards against the inside of the ratlines to the deck.
The traditional layout is to have the lines from each sail grouped together, starting from the forward end with the clewline and then the buntlines.
Some sails, in addition to clewlines and buntlines, have leechlines to pull the edges into the centre when they are stowed (see picture).