When does the permissible time for intercourse between spouses end? Is it before Fajr or before Subh?

Fajr and Subh are synonymous. However, Fajr may also refer to the first dawn (the false dawn). The time when it becomes unlawful for a fasting person to eat, drink, or engage in sexual intercourse is upon the rising of the true dawn (Fajr al-Sadiq).