Used to provide more specific description of the asset specified in SecondaryAssetSubClass(1978).
See https://www.fixtrading.org/codelists/AssetType for code list of applicable values. ISO 4721 Currency Code values are to be used when specific currency as an asset type is to be expressed.
Other values may be used by mutual agreement of the counterparties.

In the context of MiFID II's this may indicate the value needed in ESMA RTS 2 Annex IV Table 2 Field 16, or ESMA RTS 23 Annex I Table 2 'Sub product' field.