1. A system supporting use of user customizable expressions by a plurality of different executable applications, comprising: a database for associating a particular application of said plurality of different executable applications with a particular data item of a plurality of selectable data items available for incorporation in a user entered expression and for excluding use by said particular application of another data item of said plurality of selectable data items; an interface processor for receiving a user entered expression and for incorporating said particular data item in said user entered expression to provide a resultant expression for use by said particular application; and an expression processor for using data from said particular application in resolving said resultant expression to provide a result.