My DS has one from the junior senator in our state. It says Congress/Senator 1. I don't think there is necessarily an order. My guess is it means the number of nominations from that source. He is a reapplicant. Last year he got a nomination from the other senior senator. It said the same thing.
I was just responding to the explanation that "Senator 1" means the senior senator and "senator 2" means the junior senator. That was not our experience these last two years.