Update:
Contrary to everything here, vertical movements (but not pull-overs due to the internal rotation, which favors the outer chest) especially shoulder adduction like on wide-grip pull-ups, emphasize tension for the teres major + outer & mid latissimus dorsi. Horizontal pulling movements emphasize tension for the upper thickness of the lats, trapezius, and rhomboids alongside even the rear delts & rotator cuff muscles. You still only need a couple good exercises!
Bodybuilders teach that the upper back is complex to develop. They argue that you need lots of variety and should hit the muscles from many angles. If you desire a big and complete back, they claim you should differentiate between training for back thickness versus back width.
According to the lore, horizontal pulling works on thickness. Vertical pulling allows for width. You should choose dead-lifts, rows, and Olympic lifts for thickness. You should choose pull-ups, chin-ups, pull-downs, and pull-overs for width. The concept drills down further than just this though.
Close, underhand grips will emphasize thickness. Wide, overhand grips along with vigorous stretching will allow for more width. Due to the broad insertion of the latissimus dorsi, you also need to pull into many different areas of your torso to better hit specific fibers.
The Anatomy of Thickness and Width
The lats create width while the rhomboids and trapezii allow thickness.
Shoulder extension, which occurs most purely on a row, addresses many muscles. Extension itself involves the latissimus dorsi, teres major, rear deltoids, the long head of the triceps, and even the lower portions of the chest. This occurs alongside scapular retraction, which involves the rhomboids and whole trapezii.
It is clear that only the lats create width, which gets addressed with shoulder extension.
Shoulder depression, which occurs on vertical pulls, subtracts or lessens the contribution from many of these muscles but still involves the latissimus dorsi. The lower total contribution isolates the lats more so, with isolation always leading to problems. It does require a greater range of motion for them, but this is not necessarily a good thing. The range of motion for an exercise does not improve tension, the main stimulus for more size and strength. The angle of pull, or how the exercise matches up with the orientation of the muscle fibers, is lacking as well.
The other changes beyond shoulder extension and depression create more problems.
An overhand grip disables the biceps and can irritate the forearm, elbow, and wrist. This once again isolates the lats more by removing a muscle that tries to assist.
A wide grip flares the elbows. This gives a poor angle of pull for most fibers of the lats, isolating the rear deltoids and causing you to seemingly feel it more in lower lats while impinging the shoulder. A close grip that tucks the elbows shortens the lats too much and also stresses the shoulder by overemphasizing movement at the glenohumeral joint.
Many other differences that bodybuilders describe seem physiologically impossible. The upper versus lower lats divergence seems questionable given the anatomy. Could you pull on an end of a string the right way to have certain portions more or less taut?
Feel can mislead you. Many bodybuilders claim they can develop the peak of their biceps with concentration curls. They see and feel the muscle bulging up, but this occurs as the components for contraction overlap excessively and actually form less tension. This force also destabilizes the elbow.
The Row
The upper back does have several areas that each have different functions. Many of these functions should occur unloaded. Scapular elevation helps to reposition the hand, not for shrugging a heavy weight. All the muscles come together though during a basic horizontal pull.
In the past, I suggested that you could choose between a pull-up and a row. I tried to alter the pull-up so that you would assume a more horizontal position. I reconsidered the issue and can no longer recommend the pull-up in good conscience.
The row not only protects the shoulder, it places all the muscles at stronger medium lengths. It gives the best angle of pull. It addresses the lats responsible for width and all the other muscles for thickness as well. The lats will actually work harder while getting addressed with many other muscles due to concurrent activation potentiation.
On the other hand, the underhand pull-up mostly depresses the shoulder regardless of how you alter your body position. This slackens the biceps, stretches the lats, involves minimal scapular retraction, all but removes the upper trapezius, places shearing forces on the shoulders, and brings many other disadvantages. The overhead bar can irritate the wrists too.
Dead-lifts certainly work the upper back well, but fail to provide a complete pull as they neglect the elbow flexors. They have other disadvantages such as failing to provide a complete lower body movement by overemphasizing the hip extensors.
Distribute the workload evenly across all the involved parts by using medium positions. This will prevent joint issues and muscle imbalances while maximizing tension. Load and effort matter most for motor unit recruitment. Minor differences that may exist in muscle activation beyond this occur because working the muscles awkwardly will weaken some muscles that should participate. The remaining muscles are then felt more.
Your genes determine your muscle shape. Gain muscle to reveal it.
Avoid Back Thickness vs. Back Width
The length-tension relationship establishes that muscles work at stronger or weaker lengths throughout the range of motion. You work the same muscles at the endpoints but at weaker lengths and in positions that could harm the joints. Some range of motion does seem important though to allow negative work and other effects revealed when examining types of muscle contraction. This calls into question the need for many exercises to develop the upper back.
Choose a single-armed dumbbell row. Have your wrist between a neutral and underhand position. Do not flare or tuck your elbows and keep your arm relative to the body at about 45°. Make sure your wrist lines up with your shoulder and elbow as you pull. Get as strong as possible at this simple movement. Add calories to build muscle and slowly but surely see the shape of the upper back that genetics has conferred upon you.
Avoid the back width vs. back thickness distinction.