What does 1 Chronicles 5:7 mean?

The families of Reuben were recorded by their leaders, each listed by their family lines and generations. These records were made when Jotham was king of Judah and Jeroboam was king of Israel. 1 Chronicles 5:7 - Modern Text Bible

(This verse notes that the genealogies were officially recorded during the reigns of these two kings, giving a historical anchor.)