All three access modes are allowed.
In the sequential access mode, the sequence in which records are accessed is determined by the prime record key value. Records having the same duplicate value in an alternate record key which is the key of reference are made available in the same order in which they were released by execution of WRITE statements, or REWRITE statements which create such duplicate values.
In the random access mode, you control the sequence in which records are accessed. The desired record is accessed by placing the value of its record key in the RECORD KEY data item. If a set of records has alternate record key values, only the first record written is available.
In the dynamic access mode, you may change from sequential access to random access, using appropriate forms of input-output statements.