How could mdek1001 identify signal's first path

Dear sir, I read the code of TWR(DW1000)

#define RX_TIME_FP_INDEX_OFFSET (5) /* byte 5…6 16 bit First path index. /
#define RX_TIME_FP_AMPL1_OFFSET (7) /
byte 7…8 16 bit First Path Amplitude - magnitude of 3rd point after Ceiling(FP_Index) */

I wanna know How could mdek1001 identify signal’s first path. Did it use threshold to identify? If it is, how could i adjust the value of threshold?

Thank you!
Best regard.