The 3 methods are defined on the Help for the MAINT parm from CRTPF, CHGPF, CRTLF, CHGLF.
I have never had any reason to use *REBLD or *DLY.
The access path is continuously (immediately)
maintained for each physical file member. The path is
changed each time a record is changed, added to, or
deleted from the member. *IMMED is specified for all
files requiring unique keys to ensure uniqueness in
all inserts and changes.
The access path is rebuilt when a file member is
opened. The access path is continuously maintained
until the member is closed; then the access path
maintenance is ended. *REBLD is not valid for access
paths that contain unique key values.
The maintenance of the access path is delayed until
the member is opened for use. Then the access path is
changed only for records that were added, deleted, or
changed since the file was last closed. (While the
file is open, all changes made to based-on members are
immediately reflected in the access paths of the
members of the opened files, no matter what is
specified for the Access path maintenance (MAINT)
parameter.) To prevent a lengthy rebuild time when
the file is opened, *DLY should be specified only when
the number of changes to the access path between a
close operation and the next open operation are small
(when key fields in records for this access path
change infrequently). *DLY is not valid for access
paths that require unique key values.
If the number of changes between a close operation and
the next open operation reaches approximately 10% of
the access path size, the system stops saving changes
and the access path is completely rebuilt the next
time the file is opened.