There are three types vidhi vakyas, injunction statements found in the scriptures. They are apurva vidhi, niyama vidhi and parisankhya vidhi.
Apurva vidhi : when we have a new knowledge gained from a statement, and which is not gained before or / and from any other means than this type of injunction is called as apurva vidhi. The common example said is the statements like “swargakamo yajeta”, one who desires for heaven let him do the fire obulation or “vriheen proshati”, sprinkle the grain with water. In the first statement we gain the knowledge of heaven and also the means to gain it. In the second statement we gain the knowledge that the grains which are to be used for oblation should be sprinkled with water, though this sprinkling does not bring about any change in the grains; note – not all the grains as srinkled individually, and when it is done collectively not all the grains get wet;but this sprinkling gives what is called an anga apurva, partial virtue (punya).
Niyama vidhi : when there are many ways to do a karma one is prescribed which helps in ruling out / negating the other possible ways. In the karma as we saw before the vrihi, grain is to be used. From this grain the husk has to be removed to get the rice / barley / wheat etc. for the sacrifice. When there are different possibilities to do this, like – crushing the grain with a stone, removing with the nail, biting the husk with the teeth, running it through the polishing machine etc. then, the veda gives us a statement “vriheen avahanti”, let the grain be beaten to remove the husk. This statement makes it clear the method of removal of husk and so rules out the other methods.
Parisankhya vidhi : when there is a confusion of two karmas in one place or one karma for two places then accepting one and thereby negating the other is parisankhya vidhi. Example is in the karma of agnichayana, fire receiving ceremony. The clay should be brought to make the bricks ( / pot) for this we should take a horse and a donkey. The place where the hoof print of the horse is placed we should remove the clay and place it on the donkey. After collecting the necessary clay, we should bring the horse and the donkey back. Here there is a statement “imam rashanan agrbhnan amrtasya”, holding the noose string. Now there is confusion, noose of the horse or the donkey or both. To remove this confusion “iti ashva abhidaanim adatte”, with this he cathes the noose string of the horse. This statement makes it clear, only the noose string of the horse should be held thereby negating the noose string of the donkey.
Understanding these vidhi type is important not just to decipher the vedic statements. But also when there is a discussion on the type of vidhi, if any, in the statement of Vedanta, like “atma va are drshtavya shrotavya mantavya nidhidhyasitavya” oh dear, this atma is the one which should be seen, hear to, meditated upon and gain clarity of. Here the suffix (case ending) tavya is used, which denotes an injunction.