The document discusses the use of perfect modals in English. It explains that "should have + past participle" and "could have + past participle" are used to talk about unfulfilled past obligations or regrets in the past. "Could have + past participle" can also show that something was possible but did not happen in the past, suggest an alternative action that was not taken, or describe an event that almost occurred but did not. Examples are provided to illustrate the uses and rules for each modal.