In simple words, if the assets are part of the balance sheet of the central bank and fulfill certain other criteria, then they are to be treated as international reserves. If, however, the same assets are held by a long-term fund, which is a separate corporate entity, then they may not be reckoned as part of the international reserves from liquidity point of view.
However, since these assets are either under the control of central banks or can be made available to central banks, it cannot be said they are not available when needed as reserves. Basically, all foreign asset funds are owned by the public sector and, depending on the country context, these are allocated among the Stabilization Fund and SWF.