"Purpose The purpose of this Guide is to provide a practical set of guidelines for safe hydrogen use. The information presented in this Guide is intended as a reference to hydrogen design and practice and not as an authorizing document. This Guide contains minimum guidelines; users are encouraged to assess their individual programs and develop additional requirements as needed. Application The words ""shall"" and ""must"" are used in this Guide to indicate a mandatory requirement@ and the authority for the requirement is given. The words ""should"" and ""will"" are used to indicate a recommendation or that which is advised but not mandatory. The information is arranged in an easy-to-use format. The reader will find the following useful to note: ? A numbered outline format is used so information can be readily found and easily cited ? An index is provided to assist the reader in locating information on a particular topic. ? Acronyms are defined when introduced@ and a tabulation of acronyms used in the document is provided in the Acronyms section. ? Definitions of many of the terms used in this Guide are given in the Terms and Definitions section. ? All sources are referenced so the user can verify original sources as deemed necessary. References cited in the main body of the text can be found in the bibliography in Annex F. The latest revisions of codes@ standards@ and mandatory regulations should be used when those referenced are superseded. ? The International System of Units (SI) is used for primary units@ and U.S. Customary units are given in parentheses following the SI units. Some of the tables and figures contain only one set of units. "