Thanks again Bob.

Not sure what does the 'immediate' window mean...Sorry.

In your code supplied previously, the 2nd line in the Sub
Worksheet_Change body is 'Application.EnableEvents = False', which
disables events handling? I have changed it back to 'True' and tried
again but the problem still remains. Value and subsequent focus changes
among cells do not trigger any updating or other events.

Well I have to say B1 is thus populated all depending on both A1 and
A2, with A1 controls whether or not/how and A2 determines the actual
value to put into B1.