All of the data structures described in section 2.3.1 possess a header, i.e. a set of MIDAS descriptors, hereafter called the header. The header consists of two parts: the MIDAS Standard Header and the EXSAS Header. While the MIDAS Standard Header is provided by the MIDAS system, the EXSAS Header is generated and updated by EXSAS application tasks. Each header comprises an extensive set of descriptors as given below.