Where's sig from ?

[KaKi87]

Hello,
The F-Droid back-end contain sig & signer properties for each APK.
I finally figured out that the signer value can be found as Signer #1 certificate SHA-256 digest from the output of the apksigner verify --print-certs <apk> command.
However, I still can't find where to get sig from.
Does anyone know ?
Thanks