Why is the topic of sunrise always specified, and is the prayer performed before sunrise considered the Fajr prayer?

The last time for the Fajr prayer (Salat al-Subh) is sunrise. Performing the Fajr prayer before sunrise is considered an on-time performance. However, regarding voluntary (nafl) prayers after the Fajr prayer and before sunrise and the sun's elevation to the height of a spear, the Prophet, peace be upon him, forbade them, saying: "There is no prayer after the Fajr prayer until the sun rises."