The action-triggering objects Item and Button are functionally identical and both use the same trigger, the On-Select trigger. If you call a userexit at this trigger, the desired function is triggered when the button or the item is activated.
Any required parameters are passed in a separate parameter field. The parameter string may not be longer than 255 characters.
For the On_Select trigger the userexit is called in the item master, but the application parameter is passed in the menu-item relation. This allows you to use the item in several menus with the same userexit but with different parameters. So for instance you can use an item to call master lists (iwf_lis_ent), which opens different master lists in different menus.