On November 20, U.S. Customs and Border Protection (CBP) issued guidance on submitting entries containing products granted exclusions from the third tranche of Section 301 tariffs.
Instructions for importers, brokers and filers are as follows:
In addition to reporting the regular Chapters 39, 42, 44, 48, 50, 54, 60, 73, 82, 83, 84, 85, 87, 90, and 94 classifications of the HTSUS for the imported merchandise, importers shall report the HTSUS classification 9903.88.34 (Articles the product of China, as provided for in U.S. note 20(mm) to this subchapter, each covered by an exclusion granted by the U.S. Trade Representative) for imported merchandise subject to the exclusion.
Importers shall not submit the corresponding Chapter 99 HTS number for the Section 301 duties when HTS 9903.88.34 is submitted.
Exclusions granted for this are retroactive for imports on or after the initial effective date of September 24, 2018. “To request a refund of Section 301 duties paid on previous imports of products granted duty exclusions by the USTR, importers may file a Post Summary Correction (PSC) if within the PSC filing timeframe.  If the entry is beyond the PSC filing timeframe, importers may protest the liquidation.”
You may find the full guidance document here.