Secret.

 The distinction between Vidhi, Niṣedha, Veda Mantra and Purāṇa is important, especially when discussing Dharmaśāstra and Hindu practice.

1. Vidhi (विधि) — “Do this”

A Vidhi is an injunction prescribing an action.

It tells us what ought to be done.

Example: “One should perform Agnihotra.”

Its purpose is to establish a duty or prescribed practice.

In Mīmāṃsā, Vidhi is especially important because it generates an obligation to perform an action.

2. Niṣedha (निषेध) — “Do not do this”

A Niṣedha is a prohibition.

It tells us what ought not to be done.

Example: a scriptural prohibition against a particular act in a particular circumstance.

Its purpose is to prevent an action considered contrary to Dharma.

So, very simply:

Vidhi → Do this

Niṣedha → Don't do this

3. Veda Mantra — Sacred revealed utterance

A Vedic mantra is not simply an instruction.

The Vedas contain different kinds of passages, including mantras, injunctions, explanatory passages and other forms of Vedic speech. A mantra may:

invoke or address a deity,

accompany a ritual action,

praise,

pray,

contemplate,

or convey sacred meaning.

For example, the Gāyatrī Mantra is a Vedic mantra, but it is not itself a simple Vidhi such as “perform X.”

An important point is that a mantra and a Vidhi can work together in a Vedic ritual: the Vidhi prescribes the action, while the mantra may accompany or identify the ritual action.

4. Purāṇa — Sacred narrative and teaching

A Purāṇa works differently again.

Purāṇas teach Dharma through:

stories of the gods and devotees,

accounts of kings and sages,

cosmology,

pilgrimage,

festivals and vrata,

ethical teachings,

praise of particular forms of the Divine.

A Purāṇic story may illustrate or reinforce a Vidhi or Niṣedha, but a story is not automatically a Vedic injunction merely because it appears in a Purāṇa.

The easiest way to remember it

Term

Basic function

Simple question

Vidhi

Injunction

What should I do?

Niṣedha

Prohibition

What should I not do?

Veda Mantra

Sacred utterance

What sacred words are recited?

Purāṇa

Narrative + teaching

What story/teaching illustrates Dharma?

There is one further subtlety that is very relevant to Sri Vaishnava thought: not every sentence in śāstra has the same kind of authority or function. A passage that praises something, narrates something, or gives an example should not automatically be treated as a Vidhi. Distinguishing injunction, prohibition, mantra, arthavāda (explanatory/praising statement), and narrative is a major part of understanding how traditional commentators interpret scripture.

https://youtube.com/shorts/s3U75_bEyUI?si=LGahtZ5BL2Efb9xA

this is exactly the point that connects with your previous question about Vidhi, Niṣedha, Veda Mantra and Purāṇa.

The Kali-Santaraṇa Upaniṣad gives us a particularly interesting case.

The source of the Hare Rāma–Hare Kṛṣṇa mantra

The Kali-Santaraṇa Upaniṣad is traditionally associated with the Kṛṣṇa Yajurveda. It presents a dialogue between Nārada and Brahmā. Nārada asks how one can cross the difficulties of Kali Yuga, and Brahmā gives the answer: the chanting of the Divine Names. 

The sixteen names are:

Hare Rāma Hare Rāma

Rāma Rāma Hare Hare

Hare Kṛṣṇa Hare Kṛṣṇa

Kṛṣṇa Kṛṣṇa Hare Hare

The Upaniṣad calls them ṣoḍaśakaṁ nāmnām — sixteen names — and says that they destroy the impurities/effects of Kali. It further says that there is no superior means to this to be seen in the Vedas. 

There is an interesting textual point here: some recensions give the Rāma sequence first, as in your message, while others give Kṛṣṇa first. So both orders are found in circulation. 

But is this a Vidhi?

This is where our previous discussion becomes important.

The Upaniṣad does not merely give us a mantra as an isolated sacred formula. It gives the mantra in the context of Brahmā's answer to Nārada's question about the means of overcoming Kali.

So there is an injunctive/teaching force to the passage: it identifies nāma-saṅkīrtana as the prescribed spiritual means for Kali.

But technically, we should be careful about saying:

“The sixteen names themselves are a Vidhi.”

Rather, the Upaniṣadic passage teaches the means, while the sixteen names constitute the mantra/nāma to be uttered.

That distinction is quite beautiful:

Vidhi → the prescribed means

Mantra → the sacred names to be uttered

And where does Brahma Vaivarta Purāṇa come in?

This is probably what you are pointing toward.

The Brahma Vaivarta Purāṇa is a Purāṇa, and therefore belongs to a different category of śāstric literature from the Upaniṣad. If it repeats, praises or explains the chanting of the Divine Name, that does not mean the Purāṇic passage is the source of the mantra.

The Kali-Santaraṇa Upaniṣad itself is an independent scriptural source for the sixteen-name formula.

So we should not say:

“The Hare Rāma Hare Kṛṣṇa mantra comes from the Brahma Vaivarta Purāṇa.”

It is more accurate to say:

The mantra is explicitly given in the Kali-Santaraṇa Upaniṣad; Purāṇic literature also contains teachings concerning the glory and efficacy of nāma.

And this is why your earlier question was so important. A Purāṇa may praise and explain a practice; a Vedic/Upaniṣadic text may prescribe or reveal a means; and the actual sacred formula may be a mantra.

One correction to keep in mind: the claim sometimes made that the Brahma Vaivarta Purāṇa is the original source of the Hare Kṛṣṇa Mahāmantra is not the best way to put it. The Kali-Santaraṇa Upaniṣad gives the sixteen names explicitly and is therefore a very important primary textual reference. 

And I think this can become a very good article in your series:

“Hare Rāma Hare Kṛṣṇa — Mantra, Vidhi and the Scriptural Source”, explaining exactly how the Upaniṣad, Purāṇa, mantra and Vidhi differ.