An app (application) is the Boomla equivalent of classes in object oriented programming. A user space app is implemented on top of Boomla. Each user space app is stored in a file.
An app instance is a file that's type points to an app. Similar to a class instance in object oriented programming.
A built-in type is a file type that is recognized by the Boomla Platform. It may identify a built-in application, an engine, or have some special behavior attached.
The drop guide is shown during file placement operations (drag-and-drop, paste) to provide feedback about the placement. In particular, it highlights the target file and provides 3 possible placement directions: before, append, after. The color of the drop guide indicates whether the operation is allowed (typesafe) or not.
An engine is a program that executes code written in a programming language. Used as a catch-all term for interpreters and virtual machines.
The file type is a file property, holding a built-in type or an internal path to a file. Relative paths not starting with . or .. are resolved relative to the /sys/packages file. For example, the type gallery.boomla.net/gallery would resolve to /sys/packages/gallery.boomla.net/gallery.
The type chain of a file is the collection of files you encounter by recursively following a file's type property. All type chains must end in a built-in type. The type chain of a file includes the file itself.