These design constraints do not prevent dataset s from being accessed in a non-sequential fashion or by more sophisticated language s, but the format must at least work for this lowest common denominator. In addition, the length of some records depends upon information read in previous records. All of the information needed to read and decode the dataset is present in the dataset . Metadata is recorded in appropriate records in the proper record groups. Some groups are composed of only one record type, while others consist of several.
Some record types are mandatory, and others are optional.
Some types of records can be written more than once, and these will be further indexed so that they can be processed in a particular order . See Section 2.3.2 for a discussion of what indices mean in various contexts. Following the metadata come the data records themselves. The information in the previous records fully describe the structure and format of the data, making it possible to read.