So can you just tell me what are mission items how they are related to way points that we are giving
DroneCore has its own mission item representation and translates that to mavlink mission items (waypoints and commands). At this point only some of the mission functionality is supported. The aim is that it is easy to use and everything that is exposed is properly supported.
There have been questions about this before, and I don't rule out that a generic mission interface which maps 1:1 to mavlink could be added as a different plugin.
what is the use of this api (void set_current_mission_item_async(int current, result_callback_t callback))
If you use
set_current_mission_item_async() you command the drone to go this mission item. The index is relative to the DroneCore mission item vector and should not depend on the internal mavlink mission items.
and shall we send missions again 2nd time while the drone is flying using previously sent missions.
I'm not sure what you mean or what you want to do.