Advanced Database – bound control properties
Apply, remove bound control properties like: limit to list, distinct values.
Limit to list and distinct values options are only available in MS Access, not Libreoffice Base.

Limit to list changes the properties of a combo box to only allow values from the list.
To do this in Base, you simply need to select the combo box, right click and select Replace with -> List Box
Then enter or link to values in the list box as shown in a previous lesson.