The fields in a body style come in two varieties.
One variety is GLOBAL. This includes the body style notes. If you change the notes for a body style, every member of that body style is immediately affected by the change.
Other fields such as "DCC" are DEFAULTS. These fields are used when items are being created to select the default value for a field but they can be overridden. For example most engines of a certain type might be DCC-Ready but some older ones might be non-DCC and some newer ones might come with a decoder installed. Therefore it would be appropriate to set the DCC field of the body style to "Ready" but then on a case-by-case scenario, this value can be overridden.
Synchronize is used to force the default value in a body style out to all members at once. Synchronize only works for DEFAULT body style fields as all DEFAULT fields are always synchronized.