The INS (Inertial Navigation System) subsystem specifies which position and velocity estimation algorithm you are using.
Most of the INS filters are only providing position and speed, and they need to be used together with an AHRS filter for attitude. Currently, only the experimental invariant filter is a full INS.
Currently possible AHRS subsystem types are
e.g. for the extended kalman filter:
<firmware name="fixedwing or rotorcraft"> ... <subsystem name="ins" type="extended"/> </firmware>