A: We use Could 'Should have + past participle' can mean something that would have been a good idea, but that you didn't do it. It's like giving advice about the past when you say it to someone else, or regretting what you did or didn't do when you're talking about
Example :
1. I should have studied harder! (= I didn't study very hard and so I failed the exam. I'm sorry about this now.)
2. I should have gone to bed early (= I didn't go to bed early and now I'm tired).

B: We can also use should have + past participle to talk about something that, if everything is normal and okay, we think has already happened. But we're not certain that everything is fine, so we use 'should have' and not the present perfect or past simple. It's often used with 'by now'.
Example :
1. His plane should have arrived by now (= if everything is fine, the plane has arrived).
2. John should have finished work by now (= if everything is normal, John has finished work)