Iam trying to export the planned work Items under an iteration plan
from the Planned Items Page in exactly the same
sequence/order as it appears on the
page.

Can I get some code examples which show how the sequence/order of the
planned workItems is being created or associated with the planned
items..

The work items on the planned items page are essentially sorted by an
attribute such as priority and descending work item id.

Additionally, there's the custom sort mode which stores the ordering of
the work items on a plan itself. This information is stored in an
internal format and not accessible from the outside.

New with version 2.0 is a "ranking mode" (see the backlog plan mode)
which stores the exact ordering as a custom attribute together with each
work item. you can use this attribute to determine the order of work
items as seen in the backlog mode.
Enhancement 78711: "Allow work items to be sorted by ranking" will allow
to sort work item queries by "rank" (which could then be used to export
the work items as CSV for example).

Note that APT has no API that allows to access the sequence of the work
items.

--
MikeS
Jazz Agile Planning team