The cable seated row is a back thickness builder that pairs well with a vertical pulling movement like a lat pulldown or pull up, since the two together cover both the width and depth of the back. Program 3 to 4 sets of 8 to 12 reps, and rotate attachments over time: a wide bar biases the upper back and rear delts, a close V-grip emphasizes the middle back and lats, and a rope allows a deeper squeeze at the finish.
Twice a week is a solid frequency for most lifters chasing back growth, ideally spaced a few days apart. Since the seated position removes lower back involvement almost entirely, this is a good exercise to lean on when your lower back is fatigued from deadlifts or bent over rows but your upper back still needs volume.
Yes, though the difference is one of emphasis rather than a completely different exercise. A wider grip pulls the elbows out and recruits more rear deltoid and upper trap, while a narrower grip keeps the elbows closer to the body and emphasizes the lats and mid-back more directly.
The cable version keeps tension on the back throughout the entire rep and removes lower back and grip fatigue as limiting factors, which lets you focus purely on the pull. The barbell row demands more total-body stability and typically allows heavier absolute loads, so both have a place in a complete program.
There is no universal number since it depends heavily on the cable stack and pulley ratio of your machine. Judge by feel: you should be able to complete every rep with the shoulder blades pulling together and control on the return, not by matching a number you saw someone else use.
Log your cable seated row sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.