Really? How long ago was this implemented? Last time I was at Yosemite I just bought a pass on the way in and drove all the way in to see things. Is this not the case anymore?
The parent commenter is referring to the Half Dome permitting system. There's no limit on people entering the park daily (which is why it's so incredibly crowded). They do cap the number of people allowed to ascend Half Dome each day.
There's a lottery system, then a small number of tickets are given out on a first come first serve basis from the ranger stations on the day of.
