In sequential access mode, records are accessed in the ascending order of the relative record numbers of those records which currently exist within the file, except that, if the program specifies the PREVIOUS phrase, records are accessed in the descending order of the relative record numbers of those records which currently exist within the file.
In random access mode, you control the sequence in which records are accessed. The desired record is accessed by placing its relative record number in a relative key data item.
In dynamic access mode, you can change at will between sequential access and random access, using the appropriate forms of input/output statements.