Accuracy vs Accuracy-Flip

Thank you for sharing the project.
I ran a verification on a pre-trained model as written in the README.md file:

$INSIGHTFACE/src/eval/verification.py

and the scores:

[agedb_30]XNorm: 22.654594
[agedb_30]Accuracy: 0.00000+-0.00000
[agedb_30]Accuracy-Flip: 0.98250+-0.00712
Max of [agedb_30] is 0.98250

Can you explained what is the difference between the scores & especially between Accuracy and Accuracy-Flip?