What is the Choosing?

The Choosing decides who lives and who goes to the gas chambers.

"What is a Choosing?" Hannah asked Rivka out of the side of her mouth as they waited beside the cauldrons they were cleaning.

Rivka hissed Hannah quiet and ran a finger across her own throat, the same signal that the peasants had made in the fields when the cattle cars passed them by.

It's where they choose who is to be killed(In the gas chambers) next.