attitude plot data source

[vtonto]

The python plot show two euler angle sources : onboard and estimate. The estimate data comes from EKF, onboard data comes from ATT.txt, I thought the euler angle onboard data should come from the primary calculation of _ekf->UpdateStrapdownEquationsNED() function, but this function WriteFilterOutput() shows onboard data is from ATT.txt, so I am confused, what sensor data could this ATT.txt represents?