The additional surveillance initiative is part of SEBI and the Exchanges initiative to enhance market integrity and safeguard the interest of investors.

There are 2 sections of Additional margins - 

• Long term additional surveillance measures
• Short term additional surveillance measures

Refer to this FAQ from NSE. This covers the criteria based on which stocks are moved to ASM and also the surveillance actions applicable to these stocks. 

The shortlisted securities shall be further monitored on pre-determined objective criteria and would be moved into Trade for Trade segment once the criteria get satisfied.

• 100% of the traded value will get blocked as margins i.e no intraday leverage is given (MIS/CO isn't allowed)
• You can't pledge stocks which are in ASM To know the stocks which are currently under ASM, check out this page on the NSE website.
  
• Corporate actions aren't impacted by a stock being under ASM. All corporate actions like Bonus, dividend, stock split, etc. the benefits are passed on to the shareholder even though scrip is under ASM.

Note: In cases where a stock you have pledged is moved under ASM, you will no longer be provided collateral margins for that stock, because, as per ASM 100% margin should be levied. The collateral value (shown on the trading terminals) will reduce by the value of collateral received against that stock.

You can either un-pledge the stock or keep the stock pledged without collateral until the stock is moved out of ASM.