[Jhinta]

wat is inst 13 and 14 and cf 

think i found it
CF = Complemetary Filter "Attitude Estimation".
ins13 = Kalman Filter - > 13 dimensional system state

if so why ins13+cf

doesnt ins13 by default has cf already?
then what's different?

[TheOtherCliff]

Ins13 is a 13 variable EKF (enhanced kalman filter).  For instance 6 of the variables are 3D position plus 3D rotation.
Ins14 is a 14 variable EKF, it adds automatic trimming for a sensor, I forget which, maybe yaw gyro.
EKFs converge to an answer that in some sense is the best mathematical guess at the state (i.e. where the quad is and where it is pointed) given the current (noisy) sensor readings and a simplified (linear) model of how the system works.

Basic (the complementary filter) is a simpler way of calculating some of the state, and can be done on a slower CPU, but it can be fooled, for instance if you fly a continuous banked coordinated circle, it eventually thinks it is level when it is actually banked.  This and other similar issues make it unsuited for autonomous flight.

[Jhinta]

i do get that but why is there a option ins13+cf?

to me if i read wiki about it , it seems like ins13 already have cf
lets say , ins13 corrects cf with mag,gps,bias
so why ins13+cf option ?
or do i understand this wrong?

or is it depdending on a mode its in,  cf mode or ins13 mode, like flight mode setup?

[TheOtherCliff]

I haven't looked at the source, but I would guess the +CF is either for when INS13 fails, such as when mags are not good, or for manual flight modes.  Even in 16.09 there is an emergency fallback for INS13 that drives the quad at a fixed forward pitch angle and uses GPS motion direction.

You could search for the tracking issue number that did this feature, and try to read the description there.