An order is a list of one or more groups to be tested against application source code, so that the appropriate group for a build can be determined.
Whereas a buildpack group, or group, is a list of one or more buildpacks that are designed to work together. For example, a buildpack that provides node and a buildpack that provides npm.
During the detect phase, an order definition for buildpacks and an order definition for image extensions—if present—MUST be resolved into a group of component buildpacks and a group of image extensions.
A composite buildpack is a buildpack that doesn’t contain any /bin/detect or /bin/build executables; instead it references other buildpacks in its buildpack.toml via the [[order]] array. Composite buildpacks MUST be resolvable into a collection of component buildpacks. That is, after the detect phase of the lifecycle has completed, a single group of component buildpacks from the [[order]] array will have opted in to the build.
Most advanced buildpacks aren’t actually a single buildpack, but instead a composite or an ordered list of component buildpacks with each buildpack performing specific jobs. This is useful for composing more complex detection strategies.
For more details on composite buildpacks, buildpack groups, and order resolution, see the concept page for buildpack groups.