The User Interface (UI) includes messages, dialog boxes, online manuals, audio output, animations, windows, help text, tutorials, diagnostics, clip art, icons, and any presentation control that is necessary to convey information to users. The UI also includes information that is used to verify user responses, such as synonym lists that are used in command parsers.

The presentation control information (PCI) is the invisible set of controls that determine the presentation attributes of the information, such as, color, intensity, loudness, and window size.

For clear communication between the user and the application, the application must include its UI in a language that is understood by its users. Usually the application requires the UI to be translated into many languages. The cost of translation is directly proportional to the difficulty the translator has in determining the original text meaning or concept. The difficulty is compounded further when the translator must re-express the same concept within severe constraints. Such restraints can be physical or grammatical, or both. In either situation, the translation process is slowed considerably, and even halted in extreme situations.