We will update the corresponding section in the documentation to more specifically delineate the requirements for item names, slice names, and alias names. We propose wording similar to the following: Item names SHOULD follow FHIR naming guidance . All item names MUST be between 1 and 255 characters. Item names MUST begin with an uppercase ASCII letter and contain only ASCII letters, numbers, and underscores (“_”). By convention, item names SHOULD use PascalCase (also known as UpperCamelCase) . Slice names and local slice names for extensions SHOULD use lower camelCase . Slice names MUST contain only ASCII letters, numbers, underscores (“ ”), hyphens (“-”), at signs (“@”), and forward slashes (“ ”, used only for re-slicing). These conventions are consistent with FHIR naming conventions, aside from disallowing square brackets (“ [” and “] ”) since they are used by slice array path syntax. Alias names SHOULD begin with a dollar sign (“$”) and MUST otherwise contain only ASCII letters, numbers, underscores (“_”), hyphens (“-”), and dots (“.”). Beginning alias names with $ is a good practice, since this convention allows for additional error checking ( see Defining Aliases for details). As noted in the proposed text, item name and slice name requirements are based on the requirements of the FHIR specification. Alias name requirements were clarified based on analysis of use in existing FHIR Shorthand projects.