Eigentlich ist es im Handbuch gut beschrieben, s.u.
Aber das habe ich vor >10 Jahren geschrieben, jetzt probiere ich da nicht noch mal rum...
Und wenns bei Dir klappt, würde ich es auch in Ruhe lassen.


ASSUME (Assume) keyword for display file
This record-level keyword specifies that the i5/OS operating system is to assume that the record is already shown on the display when the display file is opened.
Such a record can also be defined, with the KEEP keyword, in another display file. That other display file can be closed before this file (in which you are specifying ASSUME) is opened.

KEEP (Keep) keyword for display files
You use this record-level keyword to keep the display from being deleted when the display file is closed.
The entire display is kept if any of the records on the display have KEEP specified. The default causes the entire display to be deleted when the file is closed.
In addition, the name of the first, uppermost record on the display that has the KEEP attribute is saved
by the i5/OS operating system for possible use by subsequent programs. The name kept can be used by a subsequent program that does not specify a record name on its first input operation. This keyword enables you to leave data on the display for review after your program ends, or use that data as input for subsequent programs.

Quelle: System i Programming DDS for display files Version 6 Release 1