Books published by ISBN in Hong Kong are not managed and censored by the General Administration of Press and Publication of China, so they cannot be sold directly in the Mainland. Due to the implementation of the "one country, two systems" policy in China, the publishing industry is not under the management of the General Administration of Press and Publication in the Mainland, and it is illegal to directly publish books with ISBN in Hong Kong for sale in the Mainland. If books with HKISBN want to be sold freely in the mainland market like mainland books, it is necessary for publishing management departments at or above the provincial level to review the contents of the books, make sure that the contents of the books conform to the mainland publishing policies, and then put them on record in the mainland press and publication departments, and then apply for import from China Customs through book import and export companies and sell them in the mainland. Of course, certain taxes and fees must be paid in this process. Therefore, if books are to be sold in the mainland, it is best not to apply for ISBN in Hong Kong, but to apply for ISBN in the mainland directly. Then if it's just a souvenir, a collection or a gift to relatives and friends, the Hong Kong Book Number is also a very good choice.