When defining a FIX specification, whether in a document or a machine-readable form, it's often desirable to group sets of related fields together. FIX addresses this requirement with the concept of FIX components. Components are fragments of a message that are defined once and then referenced by ID in the messages where they're utilized.
Components simplify the maintenance of FIX message definitions since changes can be made in a single location.
Components can reference other components. Circular dependencies are possible, so they should be appropriately managed in implementations.
FIX's StandardHeader and StandardTrailer are components that apply to every message.