What must you remember when you see this traffic sign?

You answered correctly. This blue circular traffic sign means that snow chains are mandatory for vehicles, and when using snow chains, the maximum allowed speed is 50 km/h. These rules ensure safety on icy or snowy roads. The sign does not give priority to track-vehicles, so that answer is incorrect. Remember, when you see this sign: snow chains must be used, and you must not exceed 50 km/h.