Count of reads F1R2 and F2R1 annotations

Hello,

From the Mutect2 output VCF, how does one interpret the values for the annotations of F1R2 and F2R1?

In the example below for a given position in a VCF:
chr5 32902979 A C GT:AD:AF:ALT_F1R2:ALT_F2R1:FOXOG:QSS:REF_F1R2:REF_F2R1 0/1:141,56:0.286:19:37:0.661:2825,403:56:85

the values are
ALT_F1R2: 19
ALT_F2R1: 37
REF_F1R2: 56
REF_F2R1: 85

Thanks for your input!
~MW