When a Reference field is configured with the Choice property "Dropdown with --None--" this will mean that the field will be displayed as a choice list such as this (when the number of items is less than 25):
If the number of items is greater than 25 then the reference field is displayed as following:
and to select the item either use
a) the suggestion list:
b) or the picker popup:
Release or Environment
Please see following documentation "Display a reference field as a choice list":
"The glide.ui.max_ref_dropdown system property determines whether a reference field is displayed as a choice list or a reference icon. When the number of available choices exceeds the value of this property, a reference icon displays instead of a choice list. The default value of this property is 25. This property affects the entire instance, however you can override this property on an individual field by using the max_ref_dropdown dictionary attribute. Modifying dictionary attributes requires the personalize_dictionary role.
The glide.xmlhttp.max_choices system property determines how many choices display on a choice list. This property has a value of 15 by default, however, when the property is not present on the instance, the instance uses a hard coded value of 10."