The XIB file format is an XML-based file format used by Apple's Xcode development environment, specifically with Interface Builder. These files are used to define the user interface (UI) elements, layouts, and interactions for applications developed for Apple platforms, including macOS, iOS, watchOS, and tvOS. A XIB file essentially stores the visual design of a single view controller, window, or custom component. Developers use Interface Builder (or code) to visually arrange buttons, text fields, labels, and set their properties, constraints, and connections (outlets and actions) to the underlying application code (usually written in Objective-C or Swift). While modern development often favors using Storyboards (which combine multiple XIBs into a single file), XIB files remain crucial for defining standalone, reusable UI components or for legacy projects. When an application is compiled, these XIB files are converted into a binary format that the operating system can load efficiently at runtime to construct the application's interface.