Circuit - an administrative division in Ancient China. At this specific time, there were ten of them, and I’m not sure to which degree they were administrative then, but originally they were purely geographic then later gained more and more administrative power.

It’s important to note that “circuit” can mean very different things during different periods and can actually be a translation of different characters (not quite indicative of which kind of curcuit is meant). Different dynasties introduced and reintroduced the concept, from the Han dynasty and to the Republic of China, so you might want to double-check if you come across the term again in a different time period.

Yangzhou and Youzhou are prefectures, also administrative divisions but lower-level. This stuff changed a lot over the centuries - in English, the same words can be translated differently to indicate this change. In Tang dynasty the hierarchy of administrative divisions goes circuit - prefecture - county. In Han dynasty, for example (I have a post coming up on my main blog), it goes province (same character as “prefecture”) - commandery - county. So… yeah, it gets complicated.

Tujue - a Turkic ethnic group