There are a number of circumstances that can 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, we will allow the client to close an existing position, but will not accept orders for that instrument which either serve to open a new position or increase the existing position. This limitation may be instrument-specific, and you may still be able to open or increase a position in another instrument.