Indian grocery shelves carry the same legume in several forms that look like different ingredients. They are processing states of one pulse, and each behaves distinctly enough to belong to different dishes.
The skin controls how fast water gets in
A whole pulse is wrapped in a seed coat that resists water, which is why whole legumes need soaking and long cooking to soften.
That coat is largely fibre and it holds the pulse together, so whole legumes keep their shape through extended cooking and suit dishes simmered for hours.
Removing it lets water reach the interior immediately, which cuts cooking time sharply and allows the pulse to break down into a purée.
Splitting exposes the interior
Splitting a pulse doubles the exposed surface and removes the barrier the intact seed presented.
Split and skinned pulses therefore cook fastest of all and collapse readily, which is exactly what a smooth dal requires.
Split pulses that keep their skin sit in between, softening reasonably quickly while retaining some texture and a darker colour in the finished dish.
Ground forms are a different ingredient entirely
Milled into flour, a pulse stops behaving like a legume and starts behaving like a starch and protein powder.
It thickens liquids, forms batters that set when fried, and binds mixtures, none of which the whole or split forms can do.
Soaked and ground to a wet paste rather than a dry flour, the same pulse gives yet another set of possibilities, including fermented batters and fried dumplings.
The forms are matched to cooking vessels and times
A household cooking on a single burner benefits from a form that softens quickly, which is part of why skinned split pulses dominate everyday cooking.
Whole pulses suit long simmering or pressure cooking, where the extended time is available and the shape is wanted.
Regional preferences track this closely, with the pulses that hold shape appearing in dishes served alongside bread and the collapsing ones in dishes served over rice.
Substituting between forms changes the dish
Using a skinned split pulse where a whole one was intended gives a dish that thickens too fast and loses the distinct pieces the recipe assumed.
Going the other way leaves firm, separate legumes floating in thin liquid instead of the smooth body the dish depends on.
Recipes that specify the form precisely are not being pedantic; the form is doing more work than the choice of pulse itself.