This is the maximum proportion of mobile traffic that this URL can receive, if provided. Therefore, you would just put 30 if you wanted a URL to receive no more than 30% mobile traffic. You would input 0 if you wanted to receive no mobile clicks at all.
Minimum and maximum percentages are upheld click-by-click when distributing clicks.
For instance, Click Handler will balance the T1/non-T1 ratio at 90% if you have a minimum T1 value of 90% and a click occurs. This implies that even if a click were to send the URL, the ratio would not be affected.