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Unfortunately, the migration won't support using a checkin record field. Whatever field you use must be included in the order record CSV export, which is limited to order record or bib record fields. If you don't have any useful fields in the order record itself that represent frequency, you could consider exporting the bib record 891|w or a similar field in your order record create lists export. If you decide to do this your Field Mapping form orders tab would look similar to the following:
Then on your Migration Form PO Claim tab, you would enter in the values that are contained in the exported MARC subfield, and the corresponding days that these values represent, like so:
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