I spell it bus because it's short for omnibus. It has to do with combining multiple signals into one, or distributing one signal to multiple places. A submix group bus is a bus even if it goes only to the main bus. It doesn't need its own output to be a bus. Insert sends aren't really buses. Direct outs, maybe they are barely buses since they split a signal two ways, internally and externally. But 99% of the time people are referring to submix buses or effects send buses.