There are a number of conditions which change the status of a product that at one time was freely tradable to later being restricted to closing only.
One of the most common causes is a change in a stock's listing that causes it to no longer be continuously quoted on an electronic market.

Another is a change to the clearing status whereby the stock can no longer be settled or cleared through standard clearing mechanisms, but rather requires special broker-to-broker bilateral settlement.

In these circumstances, MEXEM will allow the client to close an existing position, but will not accept orders which either serve to open a new position or increase the existing position.