Zhang Su is a chef and the host is a guest. What does this mean?

This means that Zhang Su is Suzaku's backpack, the kitchen of heaven, and is responsible for divination and banquets.

This sentence comes from the official book of Sima Qian, a historian of the Western Han Dynasty. The original text:

Vitamin bird note, main wood grass. Seven stars, neck, official. Major emergency. Zhang and Su are the chef and host. The wing is feather purlin, the host and guest.

Vernacular translation:

Liu Su is Suzaku's mouth. He is in charge of vegetation. The Seven Stars is Suzaku's neck, a round tube like a throat, which is used to predict emergencies. Zhang Su is Suzaku's backpack, the kitchen of heaven, and is responsible for divination and banquet. Wing lodge is the feather purlin of Suzaku, which is mainly used for divination for distant guests.

The background of "TianGuanShu":

The Official Book of Heaven is recorded in historical records. In order to know the stars and study celestial bodies, ancient people artificially divided the starry sky into several regions long ago. China called it the Star Officer and the West called it the Constellation. In ancient China, the sky was divided into three walls and twenty-eight lodgings, and the earliest complete written record can be found in Historical Records Tianguanshu.

Historical Records divides the whole day into five palaces: the "middle palace" refers to the celestial area around the North Pole, symbolizing the central court represented by Emperor Tiandi; The four palaces of East, South, West and North are built around the bisector line to Xingguan, corresponding to the four seasons, symbolizing the Four Seasons Palace visited by the Emperor of Heaven.