The use of '__user' in the Linux kernel

For example,int setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs),it makes sense to me.But I can't understand something like this,SYSCALL_DEFINE3(sigaltstack, const stack_t __user *, uss,stack_t __user *, uoss, struct pt_regs *, regs).Why there's a ',' between 'const stack_t __user*' and 'uss'.Through the body of this function, I can conclude that 'const stack_t __user' modefy 'uss'.I've learned that '__user' is a macro