Dette gælder dog ikke de obligatoriske attributter , der er tilknyttet på observa ­ tionsniveau . De knyttes til hver enkelt observation og indberettes ved alle data ­ overførsler .
Attribute values are exchanged only when they are set for the first time and whenever they change with the exception of those mandatory attributes attached at observation level , which are attached to each observation and reported at every data transmission .