There is a picture on the porcelain, next to which is written "The Sea of Yuan People". What is the source?

It should be "the law of yuan people" (that word is "law", not "sea")

Writing this was a stylized signature at that time, mainly to tell others that the painting on the jar was modeled after the painting method of Master Huang in Yuan Dynasty.

Huang was a painter in Yuan Dynasty and a native of Changshu, Jiangsu. It turned out to be the bookkeeper of the cheap visiting department in western Zhejiang, and was framed and imprisoned for being involved in the case. After he was released from prison, he traveled around the world, entertaining himself by writing poems and selling paintings, and making a living by divination. When he paints landscapes, he always observes them personally, that is, we call it sketching on the spot, and he draws Qian Shan Wangu, which is wonderful. His brushwork was first learned by Dong Yuan, Ju Ran and others in the Five Dynasties and the Early Song Dynasty. Later, influenced by Zhao Mengfu, he made good use of wet pen and became a fine pen, which was highly praised by painters in the Ming and Qing Dynasties and the Republic of China. . . At the same time, Huang's painting method was widely spread among folk craftsmen, so in the Qing Dynasty, there was a phenomenon that "imitation of Yuan people's law" or "Yuan people's law" fell on porcelain.