Arepa flour is a refined, pre-cooked corn flour (“harina de maiz refinada, precocida”.) It is not the same as the masa harina that is used in Mexico; for arepa flour, large-kerneled corn is used that has large, starchy endosperms, making for a starchier flour than that which is used for tortillas.
There are white and yellow varieties of the flour, because the corn used may be yellow or white.
Traditional process of making the flour by hand is very labour intensive. The corn is boiled in water containing lime (the chemical, not the fruit) to loosen the hulls, then drained, placed in a large wooden bowl, moistened if the time between the boiling step and this one has been long, and pounded with a wooden mallet until the hulls (with the attached germ) came off the kernels. Then it is swished in water to wash the hulls away. What is left of the kernels is then cooked, then ground (while still wet) to make it into a dough.
The dough is used straight-away. If you want to store the ground-up kernels instead, you would dry it into a flour.
Commercially now, the corn is hulled dry. The hull is first removed from the seed, as well as the germ. Then the kernels are cooked, ground, dried and packaged.
Arepa flour is sometimes referred to as a “pregelatinized” flour because the grain has been cooked first.
The process lowers the nutrients in the corn, including the protein (by about 50%), but at the same time makes what protein is left slightly more available to the body.