In one line
An option chain is one row per strike with the calls on one side and the puts on the other, and reading it well is mostly a matter of knowing which two columns to look at first.
How it works
The strikes run down the middle in order. For each one the board carries the last price, the change in price, the open interest, the change in open interest and — where the desk could compute it — the implied volatility. Calls sit on one side of the strike column and puts on the other, so a row is one strike seen from both directions.
Start with the two heaviest strikes. The call strike with the most open interest and the put strike with the most are where the largest written positions sit, and the desk marks both. When the heaviest strike falls outside the band on screen the desk pins it at the edge with its distance rather than dropping it, because losing the number the screen was opened for in order to keep the row count tidy is the one failure mode of a fixed band. Where the maximum is SHARED between strikes the desk reports no heaviest strike at all, since a tie names no single level.
MAX PAIN is the strike at which the total value owed by writers across the whole chain is smallest. It is an accounting identity over today's open interest, computed and drawn in open-interest units rather than rupees because the lot size has changed twice inside this archive's lifetime and the minimum sits at the same strike either way.
What it is not
The board is a photograph of positions, not a plan. Max pain is not a destination and the heaviest strike is not a wall — both were tested on this desk's own archive and neither said anything about where the index went. What the chain tells you reliably is what things cost and where positions are, and those are worth knowing on their own.