The highest value of ExpressionIndex ever assigned on this system. Preferrably this value is preserved across system reboots. A managed system that is unable to store expressions across reboots need not preserve this value across reboots. If all expression-creating applications cooperate, they may use this to avoid reusing an ExpressionIndex. To do so, attempt creation of a new entry with this value + 1 as the value of expExpressionIndex. Although reusing ExpressionIndexes could lead to an application receiving a misunderstood value, it is a matter of local management policy whether to reuse them.