Why have you used the red font?
Please use the normal black color as anyone else.
Regarding your question, the A models are automotive versions, they are functionally identical but they can work with the full speed in lower voltage too and also have lower consumption.
For example
MEGA16/16L
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Operating Voltages
– 2.7V - 5.5V for ATmega16L
– 4.5V - 5.5V for ATmega16
Speed Grades
– 0 - 8 MHz for ATmega16L
– 0 - 16 MHz for ATmega16
Power Consumption @ 1 MHz, 3V, and 25°C for ATmega16L
– Active: 1.1 mA
– Idle Mode: 0.35 mA
– Power-down Mode: < 1 µA
Mega16A
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Operating Voltages
– 2.7 - 5.5V for ATmega16A
Speed Grades
– 0 - 16 MHz for ATmega16A
Power Consumption @ 1 MHz, 3V, and 25°C for ATmega16A
– Active: 0.6 mA
– Idle Mode: 0.2 mA
– Power-down Mode: < 1µA
Alex