## 1 Introduction
### Countries Developing CBDCs and Types of CBDCs  
Numerous countries are developing CBDCs through pilot projects, and some of those countries have already introduced them as the national legal tender. Sweden is one of the countries currently considering adopting CBDC, E-krona, to decrease dependence on foreign electronic financial systems, such as Visa and Mastercard. The cash in circulation has fallen to 1% of its gross domestic product (GDP) [22; 23; 24; 25]. Uruguay is also considering introducing CBDC, E-peso, to overcome the underdeveloped infrastructure issue in electronic financial systems [26; 27]. The Bahamas is one of the countries that have already adopted CBDC. In 2020, the government of the Bahamas introduced the Sand Dollar, and it boosted digital transactions between people using smartphones, regardless of the existing electronic financial systems [28]. The Republic of the Marshall Islands, where the US dollar has been the public currency, issued a CBDC named Sovereign (SOV) to reduce the dependence on the US dollar and activate cross-border transactions [29; 30]. China introduced the world's largest CBDC system, E-CNY (digital yuan). To date, the number of digital yuan wallets exceeds 123 million, and the accumulated amount of E-CNY used in transactions is almost CNY 56 billion [31]. The Cambodian government and the Nigerian government also introduced CBDCs named Bakong and E-Naira.  
The CBDC systems that various countries are testing or already using take various forms depending on the purpose and can be classified according to three criteria [26]. First, the CBDC systems can be classified into the single-ledger-based system and the distributed-ledger-based system, according to the number of entities that manage the ledgers. Under the single-ledger-based system, it is the central authority, such as the central bank alone, which manages the ledger, and the central organization can manage the whole financial system with a uniform standard. However, this type of CBDC system can become vulnerable if the central authority is accidentally or maliciously shut down [32]. Under the distributed-ledger-based CBDC system, a concept opposite to the single-ledger-based system, transaction records are consensually shared and synchronized through a ledger that multiple entities can access [33]. The distributed-ledger-based system provides transparency by sharing the authority of managing among multiple entities and helps reduce the possibility of a concentrated digital attack through the distributed structure [34]. Second, the CBDC systems can be classified into the permissioned system and the permissionless system depending on whether only authorized entities can access the ledger or not. The permissioned CBDC system is based on a distributed ledger that is not publicly accessible. In this system, only users authenticated through certificates or other digital means can access the ledger and perform specific actions approved by the managing authority [35]. The permissionless CBDC system refers to a CBDC in which all entities can use the system without special permission. The permissioned CBDC system is regarded as faster and more efficient than permissionless CBDC but is not transparent and not helpful in solving the problems of financial exclusion [36]. Third, depending on whether the central bank directly provides financial services, the CBDC systems can be classified into the direct CBDC system and the indirect CBDC system. In the direct CBDC system, the central bank provides financial services to users, and all transaction records are managed by the central bank. On the other hand, under the indirect CBDC system, the central bank delegates the managing authority of financial systems to intermediaries, and the central bank only manages the intermediaries [37]. The various forms of CBDCs mentioned so far are expressed in Figure 1, and following the classification criteria, the CBDC systems currently in operation are categorized in Figure 1 and Table 1.