Dear @Chuancheng.Shi,
ElastoDyn is only coupled to HydroDyn within OpenFAST through a single 6 degree of freedom rigid body.
SubDyn is coupled to HydroDyn within OpenFAST through many degrees of freedom (depending on how many Craig-Bampton modes are enabled in SubDyn), and while SubDyn supports multiple rigid bodies (called rigid-link members in SubDyn), they must be interconnected by other members such as beams so as to avoid rigid-body modes within SubDyn. (That is, SubDyn can only support elastic modes, not rigid-body modes.)
A similar question was discussed in the following forum topic: Wind wave hybrid systems.
Best regards,