What do you mean by "Southern Prison" and "Northern Prison" in ancient China?

Imperial academy was founded in the 25th year of Yuanzhizheng in the Ming Dynasty (A.D. 1365). It was changed from Tianfu School to Guo Zi School, and taught by offering wine, doctors and teaching assistants. Ming Taizu will make Nanjing its capital and rebuild Chinese studies at the foot of Jiming Mountain. In the 15th year of Hongwu (AD 1382), Guo Zi School was changed to imperial academy. In the first year of Yongle (A.D. 1403), imperial academy was established in the Ming Dynasty. In the 18th year of Yongle, he moved the capital to Beijing, with Beijing imperial academy as the capital of imperial academy. Imperial academy was renamed Nanjing imperial academy. From then on, there was a division between the southern prison and the northern prison.