DropDownByKey 
A DropDownByKey UI element provides the user with a selection list from which he or she can select no more than one entry. The UI element consists of a text field, a button, and a selection list. Any list entry already selected is displayed in the text field. When you select the push button, a list with all possible values is displayed. When using a dropdown list box, always display this UI element alongside a Label UI element (that is, an element with a label) to ensure accessibility.
explanation
Determines the explanation text. explanation is maintained by the documentation developer in the Web Dynpro Authoring environment.
labelFor
The DropDownByKey element can also be used as a label for other UI elements. You can use the labelFor property to reference to a UI element.
readOnly
Specifies whether the user can select an element.
selectedKey
Determines the value from a value set. The value is selected from the DropDownByKey list.
size
Property is deprecated.
state
Describes whether an icon that indicates a required state is displayed. state can take the following values and is represented by enumeration type WDState: normal, required.
textDirection
Enables the labels for the DropDownByKey to be read in other languages that require a specific text direction. The textDirection property can take the following values and is represented by enumeration type WDTextDirection.
width
Determines the width of the DropDownByKey. You can specify this in CSS sizes, such as em, ex, pixels or percentage values.
onSelect
This event is triggered when the user selects a list entry from the dropdown list box.