Different data sheets list the output impedance under two different conditions. Some data sheets list closed-loop output impedance while others list open loop output impedance, both designated by Zo.
Zo is defined as the small signal impedance between the output terminal and ground (see Amplifiers, Comparators, and Special Functions, pg. 1-40). Data sheet values run from 50Ω to 200Ω.
Common emitter (bipolar) and common source (CMOS) output stages used in rail-to-rail output op amps have higher output impedance than emitter follower output stages.
Output impedance is a design issue when using rail-to-rail output op amps to drive heavy loads. If the load is mainly resistive, the output impedance will limit how close to the rails the output can go. If the load is capacitive, the extra phase shift will erode phase margin. Figure 5-7 shows how output impedance affects the output signal assuming Zo is mostly resistive.