How to read it
Fair value
Spot less the present value of dividends, grown at the interest rate for the days left. With no dividends the future is spot times e to the rate times the time.
Implied dividends
NSE does not publish the dividends of the Nifty stocks to a single number. The box starts at the dividend points that make the model equal the traded future, so the gap is zero by construction. Change it to see how sensitive the gap is.
The band
A cash and carry trade buys the stocks, funded by borrowing, and sells the future. It pays only if the future is above fair value by more than funding cost over the T-bill rate, the cost of trading about 50 stocks twice, and the futures charges. The upper edge is that level. The reverse trade needs shorting stocks for a term, which retail accounts cannot do, so the lower edge is mostly theoretical.
Where it is taught
The module on the fair value of Nifty futures derives the formula and the cash futures arbitrage module walks the band; the basis and carry module has the basics.
What it leaves out
The costs are estimates you set: the cash leg cost is a percentage of the index value for buying and later selling the stocks, and the funding spread is above or below the T-bill rate. Margin, slippage, the impact of trading the stocks together and tracking error are not priced. Dividends are modelled as received half way through the life. The band ends at expiry; a trade closed early adds the cost of that. The page cannot say whether an opportunity exists in the market today, and nothing on it is live.