The first line of a trajectory file defines the number of comment records. This is followed by ``header" records.
CORD 1000 1000 0 5932 0.204548E-01 ^ type ^begin ^skip ^void ^number of atoms ^ timestep 10000.0 800.000 12Z6 ^ scale ^ offset ^ format 174 1 21 23 27 28 39 ^number of free atoms ^ ^ ^ ^ ^ ... Ids of free atomsHeader records are in turn followed by coordinate sets separated by the string CORD. The first coordinate set contains all atoms. The subsequent coordinate sets contain only the selected atoms (see, for example, the CSELection statement in Section 10.1.6).