yoma 41

role of the red thread.
identifying tag or a weight

(discussing the use of the red thread in the rituals of goat to Azazel and the red heifer. and how much red thread is needed for each, as one ritual needs a minimum amount - either to identify, or to tie together the ceder and hyssop and be significantly heavy to throw and burn within the flames of the red heifer)

eruvin 38

some ropes are used to bind and restrict. to hold down the red heifer, to hold up the sotah's clothing.

and some ropes are used to measure, to allow freedom of movement.

(measuring the shabbat boundary should be done with a 50 cubit rope made of plant material. not iron chains, even though they would not stretch. other ropes are mentioned, each with their own uses. some ropes used to tie down and restrict, but the shabbat boundary is there is allow for movement and therefore there is room for leniency)