Rectus Abdominis

The rectus abdominis originates from the pubic crest and the pubic symphysis and inserts on the xiphoid process and the costal cartilages from the fifth to the seventh rib. Its primary function is trunk flexion, that is, bringing the sternum closer to the pubis. It's also an important trunk stabilizer during heavy compound movements: squats, deadlifts, and military presses require a strong rectus abdominis to keep the spine in a neutral position under load.

The exercises that hit it directly are crunches (and their variations), sit-ups, cable trunk flexions, and leg raises. The crunch works on the upper portion, emphasizing the approach of the sternum to the pubis. The leg raise and reverse crunch work from the opposite direction, bringing the pubis toward the sternum. For complete work, include both directions of flexion. The plank trains the rectus abdominis isometrically, as a stabilizer.

One point needs clarifying: the rectus abdominis is a single muscle. There's no such thing as 'upper abs' and 'lower abs' as separate muscles. There are, however, different degrees of activation along its length depending on the exercise. The number and symmetry of the six-pack blocks depend on genetics, on the layout of the tendinous intersections, and can't be changed with training. What you can do is increase the muscle's thickness with direct work and reduce abdominal fat with a caloric deficit.