Margin – Suppose you need to purchase a share which you wish to square off the same day. That time you purchase it on MARGIN, but remember it is dangerous because if you purchase in margin means the shares you compulsorily have to square off the same day before 2:55 pm else Reliance Money will do this for you…Its better you sell them by your own because at the last moment they will sell it in market.
There is also one thing when you purchase on margin and before 2:55 pm you are not in a position to square off and you have a sufficient balance in your ledger you can convert it to delivery…

Delivery – The share which you wish to keep for a short or for a long term, then you can buy it on delivery…
Remember the shares you purchased on delivery will reflect into your Demat only after t+2 days (Date at which you Trade plus 2)

ATST – The shares which you have purchased on delivery can be square off the next day through ATST side…
ATST means Acquire Today Sell Tomorrow same as BTST – Buy Today Sell Tomorrow.
But rememeber there are also scripts which cannot be sell at the next day…
They fell into “T” or “Z”group…