Ans. To do this you can either create a new list type(again a feature) and use the listtype number for the new list in your RegistrationType property of the Customaction. The CustomAction will then show up only for the items of this list type. or You can create a new content type and then use that content type's id in your cutsomaction to bind the custom action to items of just that content type. Add the new content type to the list where you need this customaction.