Using sigaction(), c

I was doing a little reading about sigaction() (sources are from my course notes) and I'm not sure I understand this text:

The signal mask is calculated and installed only for the duration of the signal handler.

By default, the signal “sig” is also blocked when the signal occurs.

Once an action is installed for a specific signal using sigaction, it remains installed until another action is explicitly requested.

Does this mean that the default signal mask is restored after returning form the signal handler? Also, do I have to re-install the handler after using it, as if I was using signal()?

Also, there's this piece of code:

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void termination_handler(int signum) {
    exit(7);
}

int main (void) {
  struct sigaction new_action,old_action;
  new_action.sa_handler = termination_handler;
  sigemptyset(&new_action.sa_mask);
  sigaddset(&new_action.sa_mask, SIGTERM);
  new_action.sa_flags = 0;
  sigaction(SIGINT, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN) {
         sigaction(SIGINT,&new_action,NULL);
  }
  sleep(10);
  return 0;
}

So - how exactly will SIGTERM be handled? I can see that the installed handler is termination handler(), but then SIGTERM was added to the signal mask with no use of sigprocmask(). What does this mean? Thanks!

P.s. one last question: why the if statement in main()?