Lookup screens are designed to select and return instances or lists of entities. The standard LookupAction in visual components like PickerField and LookupPickerField invokes lookup screens to select related entities.

When a lookup screen is invoked by the openLookup() method, it contains a panel with the buttons for selection. When a user selects an instance or multiple instances, the lookup screen invokes the handler which was passed to it, thus returning results to the calling code. When being invoked by openWindow() method or, for example, from the main menu, the selection panel is not displayed, effectively transforming the lookup screen into a simple screen.

The controller of a lookup screen should be inherited from the AbstractLookup class. The lookupComponent attribute of the screen’s XML must point to a component (for example Table), from which the selected entity instance should be taken as a result of the lookup.

By default, the LookupAction uses a lookup screen registered in screens.xml with the {entity_name}.lookup or {entity_name}.browse identifier, for example, sales$Customer.lookup. So make sure you have one when using components mentioned above. Studio registers browse screens with {entity_name}.browse identifiers, so they are automatically used as lookup screens.