When the AP/FD vertical mode is SRS, OP CLB, CLB, OP DES, or DES in idle path, the A/THR is in THRUST mode.
The thrust target depends on the THRUST mode.
When the vertical mode is DES in idle path and the FMS requests a thrust slightly higher than the idle thrust, the thrust mode is THR DES
When the FMS requests idle thrust, the thrust mode is THR IDLE