The supply side is the seller, the demand side is the buyer.
Legal basis:
"The Chinese people's **** and National Code Article 469 parties to the conclusion of the contract, may be in written form, oral form or other forms. Written form is a contract book, letters, telegrams, telex, fax and other forms that can tangibly represent the contents contained. Electronic data interchange, electronic mail and other ways to be able to tangibly show the content, and can be accessed at any time to check the data message, is considered to be in writing.