[seL4] i have some questions about domain and thread

Hi, all first, I do not speak English well. please answer to me easily. i'm studying seL4 kernel source code. i have some questions. 1. What is domain? i read manual but i don't understand well. I hope that you explain to me in detail about the domain. 2. What is the maximum number of threads that can be created in one domain? 3. Usually there is a function to create a new thread in kernel source code. but i can't find the function in seL4 kernel source code. What is the way to create a new user thread in seL4? also, what is the way to exit user thread in seL4? thanks.