What is the basic difference b/w "critical point control" and "management by exception"? Can we say that management by exception can be understood as a point under critical point control?

In response to first part of your query, under the principle of critical point control, an organisation cannot keep a check on all the activities of the management. Thus, this technique of controlling aims at focussing on only the key result areas (KRAs) that affect the entire organisation. For example, rise in input cost would be more important than rise in stationary cost. On the other hand, management by exception is based on the belief that 'an attempt to control everything results in controlling nothing'. According to this, only the essential and significant deviations that are beyond the acceptable limit should be controlled. For example, if there is a 6 per cent rise in labour cost whereas the permissible limit is just 3 per cent, then, this should be immediately brought into the notice of the management and a 2 percent rise in the cost can be ignored. 

In response to second part of your query, yes we can we say that management by exception is a point under critical point control because in the principle of exception deviation is taken care of in respect of the predetermined critical points.