## 2 Background and Example  
### Digital Calibration Certificates  
As shown in Figure 1, a DCC comprises three parts: Norms and Standards, Hierarchical Structure, and XML as Exchange Format. The Norms and Standards must comply with international standards such as The International System of Units (SI), International Vocabulary of Metrology (VIM), Guide to the Expression of Uncertainty in Measurement (GUM), Committee on Data for Science and Technology (CODATA), and ISO 17025. Since 1960, the SI system has been the preferred system of units globally. It comprises seven base units and 22 derived units, all of which are based on natural constants. The definitions of these units are established using a set of seven defining constants, which are the fundamental feature of the entire system. VIM is a standardized set of definitions and terms used in metrology that is developed at an international level by various standardization bodies, metrology organizations, and accreditation laboratories. Its aim is to provide a clear and unique language for anyone interested in measurements. GUM provides guidelines for evaluating and expressing uncertainty in measurement that can be used for a wide range of measurements. When an instrument is calibrated, it must be traceable back to a common standard, and this requires estimating and showing the uncertainty at each step of the traceability chain. CODATA periodically issues a set of recommended values for fundamental physical constants. These values are established by analyzing all the available theoretical and experimental data using a least-squares adjustment method. The CODATA task group on fundamental constants plays a crucial role in selecting and evaluating the data to ensure accuracy. ISO 17025 is a well-recognized quality standard that outlines the necessary requirements for calibration and testing labs to demonstrate their competence, technical proficiency, and ability to produce valid results. The Hierarchical Structure has four sections: administrative data, measurement results, comments, and a document. The administrative data are required and regulated, and include fundamental information such as the name of the measurement object, calibration laboratory, reference number, and date. The measurement results are also required but partially restricted, and any legal data must be expressed in the SI units. For example, the type definition of the two tree structure elements designated as administrative Data and measurement Results is shown in Figure 2[28; 38; 39]. The administrative Data element has six mandatory child elements and two optional child elements, and the measurement Result element has six optional child elements and two mandatory child elements. The comments section is optional and unregulated, allowing the calibration laboratory to add any additional digital information they deem necessary. For instance, the human-readable document is optional but recommended, and users can be provided with a digital copy of the analogue calibration certificate for easy accessibility. In other words, the administrative data and measurement results elements are necessary components of a valid DCC. Without these elements, a DCC cannot be considered complete. On the other hand, the comments and document elements are not required and can be included at the discretion of the user. Figure 1: The schematic structure of the digital calibration certificate [9]. The XML as Exchange Format is an internationally accepted, approved, and cryptographically signable XML exchange file that is used as a data format, facilitating efficient and accurate calibration information management. In addition, the use of cryptographic protection ensures the integrity and authenticity of the data, which is essential for building trust and ensuring that the data have not been tampered with. The IEC TS 62720 standard [40] provides guidance on handling SI units and derived units in XML to ensure compliance with international standards and facilitate interoperability between different organizations [9].