This iExpression links a 3D property to another 1D, 2D, or 3D property. As in any other link of iExpressions, you can choose in the options of "Link to" many details of the link, e.g. whether to link to the actual value or to its speed and which component should be linked to the x and y component, respectively.
Don't put a number here. Use the link button to link this to some property.