Why are six prayers written in office calendars and diaries instead of five, and is Fajr prayer the same as Subh prayer, and which of them is obligatory?

Allah has given prayer a great status; it is the pillar of the religion, and it was enjoined in the heavens without an intermediary. Muslims have unanimously agreed on its obligation and that there are five prayers in the day and night, based on His saying, the Almighty: "Establish prayer and give zakat," and the hadith: "Inform them that Allah has obligated upon them five prayers every day and night." What is written in calendars regarding six prayers is by adding the sunrise prayer to indicate the time of the supererogatory prayer. The Fajr prayer and the Subh prayer are two synonymous names for one entity, and it is sunnah to pray two rak'ahs before them, which are called the Sunnah of Fajr or Subh.