The CGNS/SIDS is a data model. It describes the data you need to define (mandatory or optional data) the semantics of it, the way you organize it and its relationships with the other data. This describes a logical model of your application data.
A CGNS is a tree. A tree is a node which has node has children. The top node of a CGNS/SIDS tree has the CGNSBase_t type.
