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They are actually RDA Carrier Types and have just been loaded (actually I'll get this done tomorrow) via API with /opt/folio/api/POST/instanceFormats.pl.
When loading titles (instances) it will be necessary to look at the 338$b (or 337$a & 338$a) to determine which instanceFormatId(s) to include in the JSON doc that will be POSTED POSTed via API. If there is no 338 it can be left out of the JSON doc. - 6/45/2020