Each custom action (built-in or end user created) follows the API which supplies CustomActionContext object. The context object provides Items property which is of ICollection. In case of single selection, the first item in the collection will be used, and when multiple items are selected, all items are passed to you via context.Items. In case of GoToUrl action, the first item or row is used to generate parameter. If you are creating custom actions, you will need to handle in your code.