When defining the structure of a FIX message, whether in a document or a machine-readable form, there will often be logical groups of fields that recur across multiple messages. FIX refers to these logical groupings as components. A common example is the Instrument component, which groups all the fields related to a security or instrument's static data.
View all the components defined in FIX LatestComponents simplify the maintenance of FIX message definitions, as changes can be made in a single location. A component is defined once and then referenced by its name or an ID when defining the structure of a message.
Components can also include other components. Circular dependencies are technically possible, but should usually be avoided.
In TagValue encoding as well as other encodings, FIX messages are framed, meaning they include a StandardHeader before the message body, and a StandardTrailer to after the message body. These are typically defined as components.
Depending on the format of the FIX dictionary being used, the inclusion of the header and trailer in a message definition may be either explicit or implicit.