Offboard mode: slighty better intro

en/flight_modes/offboard.md

@@ -2,12 +2,17 @@
 
 [<img src="../../assets/site/position_fixed.svg" title="Position fix required (e.g. GPS)" width="30px" />](../getting_started/flight_modes.md#key_position_fixed)
 
-The vehicle obeys position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints provided by some source that is external to the flight stack, such as a companion computer.
-The setpoints may be provided using MAVLink (or a MAVLink API such as [MAVSDK](https://mavsdk.mavlink.io/)) or by [ROS 2](../ros/ros2.md).
+The _Offboard_ mode allows PX4 to be controlled by some source that is external to the flight stack, such as a companion computer.
+
+The offboard controller supplies a stream of position, velocity, acceleration, attitude, attitude rate, or thrust/torque setpoints, which are injected into the [PX4 control architecture](../flight_stack/controller_diagrams.md) as though they had been generated by the equivalent internal controller — the corresponding internal controllers that would otherwise have generated the setpoint (in another mode) are turned off.
 
+The setpoints may be provided using MAVLink (or a MAVLink API such as [MAVSDK](https://mavsdk.mavlink.io/)) or by [ROS 2](../ros/ros2.md).
 PX4 requires that the external controller provides a continuous 2Hz "proof of life" signal, by streaming any of the supported MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) message.
 PX4 enables offboard control only after receiving the signal for more than a second, and will regain control if the signal stops.
 
+Different vehicle frame types implement different [control architectures](../flight_stack/controller_diagrams.md).
+As a result the setpoints and setpoint combinations that can be set depend on the vehicle.
+
 :::note
 
 - This mode requires position or pose/attitude information - e.g. GPS, optical flow, visual-inertial odometry, mocap, etc.