You are the declarer at a 3NT contract (1NT, 3NT, no opposing bids). West leads what appears to be a five card suit, and you have Axx in dummy.
Your main concern is that East can get the setting trick by attacking a vulnerable tenace you have in hand; something like Jxx in the led suit, or AQx in a side suit (and maybe the the answer to the question below is found in the difference between one and the other).
At times, you should go up with the ace in dummy (if East has an honor and a small card) to "block" the suit. At other times, you should hold up the ace until the third round to exhaust East of the led suit. How do you tell the difference?
A club kibitzer gave the opinion, "You use a blocking play if you're worried about what will happen in the LED suit (e.g. if your tenace is Jxx, above), and you use a holdup play when you're worried about what will happen in a different suit (e.g, your AQx tenace)." Is there any truth to this statement?
I haven't stated whether West led from "top of a sequence" or "fourth best," but maybe the answer to question lies in the difference between the two.
Because another take of this matter is "avoidance." That is, if the priority is to keep East off lead, then let West win if he led a high card, and cover if West led fourth best, to prevent East from getting the lead.