May be used to provide a more specific description of the asset specified in UnderlyingSecondaryAssetType(2083).
See https://www.fixtrading.org/codelists/AssetSubType for code list of applicable values.

In the context of MiFID II, ESMA RTS 23 Annex I Table 2, this may indicate the 'Further sub product' or equity 'Parameter' fields.