As of Friday Russia had about $630 billion of foreign currency reserves, a large cushion designed to allow it to withstand economic sanctions and prop up the value of the ruble. But “foreign currency reserves” are not an objective fact; they are mostly a series of entries on lists maintained by foreign-currency issuers and intermediaries (central banks, correspondent banks, sovereign bond issuers, brokerages). If those people cross you off the list, or put an asterisk next to your entry freezing your funds, then you can’t use those funds anymore.

The bulk of Russia’s foreign reserves are held in the form of securities, deposits at other central banks and deposits at foreign commercial banks. A ban on transactions with Russia’s central bank means that it can’t sell those securities or access those deposits