Material Type occurs in two places: Orders and Items.
The Order Material Type should be the appropriate to the presumed type of material being ordered. It has no effect on the predicted items.
The Item Material Type should be appropriate to the material in hand, according to the table.
For Serial Checkin, Material Type is not an option to enter in the subscription record which controls items. All predicted items are ISSUE by default.
This is correct for print, but NOT correct for other formats.
You cannot change the Material Type to the correct code BEFORE checkin because if you do that, the item is removed from the list of predicted issues and you can't check it in. It doesn't matter if you try this in [K] Checkin or [M] Item list.
For all non-print serials, the Material Type must be changed AFTER checkin. This can be done in serials from [M] Item list in Acquisitions/Serials (lower pane) or in Cataloging [L] Items list (lower pane)
Material Types for Orders and Items Table (below document)