The Lookup and SelectBox widgets implement a similar functionality - while both consist of a text field and display a collection of items in the drop-down list, they also have differences:
Drop-down field content
The SelectBox has only a list of items in a drop-down field whereas Lookup also has a search field and the Cancel button.
Appearance on desktop and mobile devices
The SelectBox looks the same on any platform, while Lookup changes its appearance. The Lookup's drop-down field is replaced with a popup window that occupies most of the screen. This design optimizes Lookup for mobile devices.
SelectBox on any device Lookup on desktop Lookup on mobile devices