get index of first structure type returns: -1 if no structure type is defined index of first structure type. Each structure type has an index and ID. INDEX determines position of structure definition in the list of structure definitions. Index 1 is listed first, after index 2 and so on. The index of a structure type can be changed any time, leading to movement of the structure definition in the list of structure definitions. ID uniquely denotes a structure type. A structure gets a unique ID at the creation time and this ID can't be changed. Even when the structure type gets deleted, its ID won't be resued in the future.