What is the difference between 10800 and 12800 RAM for laptops?

pu1. I'm talking about the clock speed, but the latest laptop will work with a 12800, and the old laptop will work with a 12800, but it'll go back to 10800, so you can just buy a cheap one

2017-11-22

OK

pu2. If it's an old laptop, it'll work at 10800 no matter which one you plug it in, so you can buy a cheap one between the two and plug it in

2017-11-22

OK

pu3. If you just buy a 12800 and plug it in, it'll be 12800 if it's a laptop that supports 12800 degrees
If it doesn't work, it's automatically 10,800, so it's good to buy it with 12800

2017-11-22

OK