CMakeJSON parses the json files recursivly. The implementation can be viewed here. The function doing the parsing is cmakejson_parse_json.
For the project JSON the parsed variables are named CMakeJSON_PARSE_PROJECT(_<member|index>)+. Using index only works for arrays. As an example: CMakeJSON_PARSE_PROJECT_NAME is the root name member in the JSON CMakeJSON_PARSE_PROJECT_DEPENDENCIES_0_NAME is the name member of the first element in the root array member dependencies.
For a target JSON it is similar but the variables are CMakeJSON_PARSE_TARGET(_<member|index>)+ and only have function scope in cmakejson_add_target.