Waypoint is just a generic term for any flight point. It could be a route (flight plan) point, a mark point, an achor point, etc. A steerpoint is the waypoint you have currently selected for navigation. For example, if you have a flight plan with 6 waypoints, you will probably set waypoint-1 (usually called MSN001) as your steerpoint at the start of the mission. As you approach waypoint-1, you will select waypoint-2 as your steerpoint, etc. Maybe it would help to think of a steerpoint as the "active" waypoint.