A "security" is something that has a notional value. You can buy and sell the different types of securities on an exchange.
So it's synonymous to an asset (i.e. something of value that you are prepared to buy or sell).
The exchange does nothing more that manage the buying and selling of these securities to guarantee no one gets ripped off.
One party hands over the security, the other party hands over the cash and the exchange manages the deal on behalf of both parties (for a small fee).
Now the exchange can call a halt to trading (i.e. stops all buying and selling) when it see the market is in trouble. The exchange just calls a halt in an attempt to slow the fall of the market.
These halts are used to try and smooth out extreme fluctuations in the market.
So in this case it looks like the price of Russian securities has collapsed (i.e they are becoming dramatically worth less than they were a few days ago) and the exchange has been temporarily blocked the buying and selling Russian securities in the hope their value stabilises.
Currently the exchange is just seeing a collapse in price of the Russian securities and they want to slow that fall.
No this is not a 'normal' blocking of trading in securities due to turbulance in the market. It is still allowed to buy Russian securities. It is also allowed to Russians to sell Russian securities. The Russian exchange has specifically forbidden foreigners to sell Russian shares.
https://www.politico.eu/article/russia-central-bank-bans-sal...
So if this is the case (and I don't doubt what you are saying) then that means Russia has decided to turn it's functioning market into something that is no longer a market.
Only because we now have a situation where any foreigner holding a Russian security can no longer sell that security.
That action has instantly made the security worthless (i.e. value of zero) to the foreigner.
So while these actions might stop the price drop in the short term, it will never last only because the market created is no longer reflecting the true value of the security.
They are now only being held artificially high because no one can buy and sell them.
So it's synonymous to an asset (i.e. something of value that you are prepared to buy or sell).
The exchange does nothing more that manage the buying and selling of these securities to guarantee no one gets ripped off.
One party hands over the security, the other party hands over the cash and the exchange manages the deal on behalf of both parties (for a small fee).
Now the exchange can call a halt to trading (i.e. stops all buying and selling) when it see the market is in trouble. The exchange just calls a halt in an attempt to slow the fall of the market.
These halts are used to try and smooth out extreme fluctuations in the market.
So in this case it looks like the price of Russian securities has collapsed (i.e they are becoming dramatically worth less than they were a few days ago) and the exchange has been temporarily blocked the buying and selling Russian securities in the hope their value stabilises.
Currently the exchange is just seeing a collapse in price of the Russian securities and they want to slow that fall.