Sub-type qualification/identification of the SecurityType. As an example for SecurityType(167)=REPO, the SecuritySubType=General Collateral can be used to further specify the type of REPO.
If SecuritySubType is used, then SecurityType is required.
For SecurityType=MLEG a name of the option or futures strategy name can be specified, such as Calendar, Vertical, Butterfly.
For SecurityType(167)=OPT the subclassification can be specified, such as Asian.
For SecurityType(167)=SWAPTION a value of Straddle is used to identify a straddle swaption.
In the context of EU SFTR reporting use the appropriate 4-character code noted in the regulations - GENE for general collateral or SPEC for specific collateral (without quote marks).