Recently, the BAC Blockchain Application Conference was held at Tsinghua University. Bai Shuo, chairman of the Shanghai Branch of the China Computer Federation (CCF), pointed out in his speech that the core application of blockchain is payment exchange. Payment is one-handed payment and one-handed delivery, while exchange is the payment of different ends of the payment. Bai Shuo believes that the blockchain can use the "three-point method" to distinguish whether it is legal or not in the field of payment exchange. Digital currencies can be divided into three categories: virtual currencies, business circle currencies, and legal digital currencies. Virtual currency is private, trying to be unregulated, as a means of payment and exchange is prohibited in China, and as a means of asset valuation, it is partially formal; commercial currency can be regarded as a binding legal currency, like Meal tickets, but it is opaque in the process of creating money; legal digital currencies are led by the central bank, have legal effect, and can be circulated online. From the perspective of property attributes, virtual currency is legally recognized as a property, and theft is an offence. However, virtual currencies do not have the ability to pay, and business circle coins have the ability to pay within a specific range of the business circle, but cannot be outside the established range. Everything is legal in fiat digital currency.