Having a shape or a cross-section like the capital letter X. An x-shape as an International Association of Lighthouse Authorities - IALA topmark should be 3 dimensional in shape. It is made of at least three crossed bars.
Chapter 2, page 2.223
Definition Source :
IHO Transfer Standard for Digital Hydrographic Data, Appendix A: Object Catalogue - Description of the Feature Coding Schema to be Used for Hydrographic Requirements
On behalf of the IHO Member States, the International Hydrographic Bureau would like to thank Korea Hydrographic and
Oceanographic Agency(KHOA) of the Republic of Korea for giving active support in the development of the S-100 GI Registry