Random access files are:

•   fixed length records containing a mixture of field types

•   ISAM and ISAM-A files are special variants of random access files

•   files must be pre-allocated; see ALLOCATE, CREATE

•   file expansion typically not automatic, except for ISAM, ISAM-A; see MX_EXPFIL

•   file and record locking is available; see LOKSER

•   records may be "blocked" into 512 byte blocks; see OPEN statement

See File Access Methods for an explanation / etymology of "Random Access".