why is the start of frame a dominant bit in controller area network?

Answer 1

The start of frame bit is just that, it designates the start of a frame transmission, generally for synchronization purposes. It will always be a logical 0 which is a dominant bit. Dominant means logical 0 and recessive means logical 1. Since this is on a bus, I think the "why" part of your question is that it's dominant so that it cannot be overridden. If it were recessive then it could be overridden by another dominant bit.