Grammar 6.4: Five different ways to express obligation
Again, it's totally unnecessary for your textbook to devote a grammar section to this.
All that's being presented here are 5 different ways to express obligation.
3 verbs/verb phrases for personalized expressions of obligation (ie, the actant is the one designated as specifically obligated to perform the action):
2 verb phrases for impersonal expressions of obligation (ie, generalizations about being obligated to perform an action):