This ATC report presents guidance material for the use of "Ground-Initiated Comm. B" (GICB) register set contained in a Mode S transponder. The guidance material is intended to provide assistance for implementers of Mode S avionics installations. A common summary of the requirements and specifications for Mode S GICB transponder register data link applications is developed. While this ATC report focuses primarily on the "Elementary Surveillance" (ELS), "Enhanced Surveillance" (EHS), and "Automatic Dependent Surveillance-Broadcast" (ADS-B) applications, guidance information is also provided for general transponder configuration and architecture of other Mode S functions employing the GICB register set. Although the information contained in this ATC report is drawn from a number of approved national and international standards, it is not intended to replace or supersede those standards documents. In the event of a conflict or contradiction between this ATC report and any approved standards (see references 1 through 6), the approved standard takes precedence and the reader is encouraged to contact the authors of this document. Reference 4 is the most-recent and complete specification for the Mode S register contents. For ease of reference, the relevant Mode S register images have been duplicated in Appendix A of this report.