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I'll make this really quick and simple:

The absolute max is the highest point.  period!

The absolute min is the lowest point.  period!

Check it out:

a graph ... ( -3 , -2 ) is the lowest point and ( 2 , 2 ) is the highest point

f has an absolute max of 2 at x = 2.
(This is also a relative max!)

f has an absolute min of -2 at x = -3.
(This cannot be a relative min, since it doesn't
have points on BOTH sides to compare it to.)

Endpoints cannot be relative extrema!
 
Endpoints CAN be absolute extrema!