The ISN buffer defines an area in storage to which Adabas can return the ISNs of the records that satisfy the specified search criteria for a command. In addition, if Command Option 1 for a command is set to "M" (or "P"), assuming these are valid settings for that command, and if the command is issued using the ACB direct call interface, the ISN buffer holds the record descriptor elements (RDEs) of multifetched or prefetched records awaiting processing. If, instead, the command is issued using the ACBX direct call interface, the ISN buffer is not used for this purpose; the multifetch buffer is used instead.

When needed, only one ISN buffer should be specified in a direct call.

The four-byte binary ISNs are usually provided in the ISN buffer in ascending sequence. For the S2 or S9 command, the ISNs are provided according to the user-specified sort sequence. If the ISN buffer is not large enough to contain the entire resulting ISN list, Adabas stores the overflow ISNs on the Adabas work data set, if requested. These overflow ISNs can then be retrieved at a later time.

If the resulting ISNs are to be read using the GET NEXT option of the L1 or L4 commands, the ISN buffer is not needed.

The ISN buffer also supplies an ISN list to be used as input when the ET or BT command specifies a Command Option 1 of "M" (or "P").