It’s a very common requirement to provide a listbox in the UI forms where a user can select single or multiple values from pre-defined list. The list can be stored separately in the SharePoint list, Excel file or SQL table. Admin team should easily be able to manage that list without impacting the front end UI forms. 

In SharePoint online list, you might have columns where you want to show dropdown, where data is fetched from some other data source. In this video, we will see how we can achieve this requirement.